There is no fallacy here since a fallacy involves a logical error in reasoning and I can't see that your friend commits that.
More than that, your friend's conduct is capable of defence on grounds of actual consequence utilitarianism. This position is not open to the 'what if everyone did the same?' objection since it rejects the requirement hypothetically to universalise. He can readily grant that if everyone did the same, and uprooted plants, there would be no plants for anyone including himself to uproot. But the mass uprooting of plants is not the actual consequence of his activity in that forest at that time and so fulfils the actual consequence criterion.
There are undoubted cases where this argument breaks down, and the actual consequence is that everyone does do the same and we all lose out. But this only means that the actual consequence criterion has to be indexed to a context. Your friend can grant this but still maintain that his uprooting of plants in that forest at that time does not have the actual consequence that all the plants are uprooted.
Universalisability is inbuilt into most ethical theories but not of all - ethical particularism is an example to the contrary. But actual consequence utiltiarianism can plainly be universalised. There is no bar, logical or prudential, to everyone's applying this moral criterion. Your friend has no grounds on which to deny this but then he has also to accept that while actual consequence utiltarianism justifies his uprooting of plants in that forest at that time, in different conditions it does not. Conditions are perfectly possible in which his uprooting does have the consequence that everyone uproots and so there is nothing left for himself or anybody else, in which case he is condemned by his own actual consequence criterion.