While this argument is technically not wrong (someone else will probably do it), there are still issues.
Firstly, they seem to agree that the deed in itself is ethically wrong since the argument is not "doing it is not evil" but "not doing it is not good".
They are essentially shifting blame from themselfes to the world. But, and that is the point, what they are doing is still wrong.
I think of it as a form of the Tinkerbell effect. Because so many people believe it is true, it automatically becomes justificated.
But, reversing this, if people would stop believing it, it would become falser.
Not false, only less true. But making this less true is already good since it is wrong to do it, and we reduce the truthness of a justification.
Another way to see this is in the form of the heap paradoxon. A number of Sandgrains is called a heap if you can remove one grain, and it is till a heap. Starting from this definition any number of sandgrains is a heap, which makes no sense.
Similarily -according to your colleague- if we have several people doing something wrong, stopping one of them is not good, since any of the other ones will do it instead. But this would mean, even if we do this hundreds, or thousands of times, it would still not be good, which is obviously false, since somewhere along the line, we run out of people who would do this.
Since the sum of these acts is good, shouldn't a single one of them be good also?
You can even trip him into an inconsistent stance. Ask him if he votes.
If yes, tell him that, according to his worldview, it makes no sense to vote, because even if he didn't nothing would change (with a high probability).
If no, then you have a consistent and slightly douchy acquintance ;)
All this is summed up pretty neatly as:
Be the change you want to see in the world!