I personally never keep fish but don't really have a problem with it as long as it is legal. You are correct about the trout overpopulating. There are tons of fish in the Smokies streams but mostly they are not very big. In my opinion, if you are going to keep a few, keep ones that are at the lower end of the size limit. The harder to find 10-12 inch fish are the ones that will be passing on the "good" genes and if people keep taking those fish out, the population may become stunted over time. I do not know for sure if this is actually true or not but it is an argument I have heard and it seems reasonable enough to me. Let me warn you also that some people may be downright upset if they see your friend keeping a few fish. This is just something that you should probably be prepared for from some people that think their way is the only way. Finally, best of all just talk your friend into catch and release...