This debate is apparently based on loose terminology. Here, some definitions and ideas that might help.
Modernity is a period in history, which started with the Enlightenment and ended around 1950, postmodernity becoming right after; commonly associated with a loss of religious and moral values against individualism and scientific and technical developments. So, Capitalism is debatably a consequence of it, not a cause of it, it is not Capitalism that raised a (scientifical-technical) period in history.
Colonialism is the practice by which a territory and a human group are subject to a foreign rule; colonialism is sometimes associated with an evolved form of animal territorial domination (cf. Fanon), which raises directly from Darwinian mechanisms. According to your friends, this is the cause of all evil; the problem is easy to solve: they can just derogate all Darwinian laws in the Congress. Oh, wait. I forget there's no universal congress. And no congress which rules animal instinctive laws (we are in part animals). And no congress that accepted Darwinian laws of evolution and survival. And no congress members proposing alternatives to the survival of the fittest rule.
Capitalism means essentially the tendency to accumulation of capital, which includes the means of production, rights of ownership, free market development, natural resources, and not just that, but mainly a bunch of histories that only exist in our heads, like money, laws, power, status, etc. There is also has a good solution to this "problem", that proposed by Pol Pot, and which results are already available in history books. They can also read about Ceausescu, Mao Zedong, Stalin, Maduro, Castro, Allende, the Kim Jongs, Mariam, Hoxha, Ortega, Morales, etc. But warn them ALL cases are failures.
Science is some type of knowledge (that obtained according to the Scientific Method). Technology is the application of such knowledge in practice, and Art (not as aesthetics, but as in state-of-the-arts) is the application of those two for social benefit.
From here on, this is necessarily an opinion with some philosophical sustain. Politically, this issue is largely debatable.
Call this The Natural Test: The deepest of our instincts is to survive, and that is normally consistent with all our emotions and reason. When any living being, any individual, any human group, etc. tries a new rule, it is just tested by nature: either that being survives or not. Some specific groups that have submitted new ideas to The Natural Test and ended up with huge amounts of deaths: the FARC; the Aum Shinrikyo cult; Ugandan LRA; North Korea; Jim Jones people's temple; the Khmer Rouge; Nazism, etc.
There's no point in attacking science, tech and art. Europe developed a culture for knowledge, which is a positive achievement, not a failure. Anyway, science has been always developed, all along history, in all places where humans existed. If they don't like scientific colonialism, they just need to invent the wheel from scratch, and all remaining science and technology as well.
About “colonial technology”: just ask your friends to throw their laptops, TVs and mobile phones to a river. That way, they will be getting back their sovereignty.
Why are decolonialist attacking science and modernity?
The big philosophical issue here is responsibility.
By definition, any responsible individual takes responsibility of what happens for himself. The responsibility to address social problems, the responsibility to solve individual and social issues. Then, he acts on it, without blaming or without complaining (blaming and complaining is just energy waste). For example: are there poverty issues? Ok, propose solutions without blaming and complaining. Many organizations do so and get it successfully. Your friends need to read about Médecins Sans Frontières, Water.org, Acumen, Heifer, Grameen Bank, etc.
By definition, individuals which feel non-responsible for what currently happens, can't take responsibility; instead, they look for blame. A lot of politicians do so. This approach produces social polarization and usually gets a lot of votes. But this approach does not propose solutions. And if they have done so, they have been submitted to The Natural Test and have failed. They nevertheless insist: Pablo Iglesias (Spain), Maduro (Venezuela), Corbyn (UK), Mélenchon (La France Insoumisse), Máximo Kirschner (Argentina), Correa, Diaz-Canel...
Sorry for this, not personal, but for me, your friends are typical examples of individuals in the second group. They are looking for who to blame -don't understand what for, perhaps to kill or imprison them-. If they believe in their ideas, they need to practice them. If they don't apply them, and they provide excuses, they don't really believe them. In such case, their discourse is not based on rationality, but on dogmas, blaming, complaining, use of excuses and personal attacks. Their discourse is evidently explained by personal frustration (causes them to point their failures elsewhere) and resentment (causes them to attack others). You can justify your position, but such kind of people will always find excuses to avoid doing what they say they believe. The arguments are as follows.
Decolonialism has failed. Deglobalization has and will always fail (maturity is not about isolation; it's not about dependence, it's not about independence: it's about interdependence). Open doors are always good, even if we don't use them; we want them to be open and use them for good. The best examples are North Korea, Cuba, Venezuela and Argentina. They reject modernity (we are not anymore there, anyway, your friends can just create their own social experiments, living like in 1700), capitalism (ask them to give the example, stop accumulating capital and share all their goods, right now!). When they pay taxes, they are supporting colonialism. Ask them to stop buying foreign products, stop paying taxes, stop using the internet, stop learning foreign languages, etc. Otherwise (if your friends don't act, dont donate their goods, don't crush their mobile phones, don't go living in communist kibbutz), those are just empt y words, which just expose their frustrations and anguish of blaming other for their own fails. Not trying to be mean here, but this is a common and predictable social pattern based on dogmas, which provides good amounts of votes to the left.
As a consequence, notice that the left wing has an ideology, but most voters are not serious about it (most don't even know it, otherwise, they would act critically about it -see spanish Ramon Rallo's daily criticism to left wing ideas, which is impressive, he just wrote a book about the fallacies of Marxism-): left-wing voters are serious mostly about confronting and harming the right wing. Just that.
For me, left means: the survival of the group prevails over the individual, which is just a natural mechanism based on majority (e.g. if I kill someone, I want to live, the group wants to kill me, so, the group wins). Taking that into account, I identify myself as such kind of leftist, which do implies complete logical consistency with capitalism, globalism, Darwinian laws, science, tech and art, etc., and mainly... responsibility (no blaming, no complaining, just acting silently to solve issues).