It's very sad when people asks for advice and when it's not what they want to hear just go and do what they want at the expense of the fish.
I have a 4000g pond, a smaller pond, and kept 8 now very large goldfish that now reside with the grandkids for years. I had 4- 8+ inch goldies in each 55g tank that grew tons the first year. The ammonia levels in the tank will most likely kill them quickly as they release copious amounts of waste and ammonia that will use the buffers in the water in no time causing ph to drop and ammonia levels to continue to rise... and that's just the tip of the iceburg! Also stunting will occur very quickly which will also lead to premature death. I don't think I need to go on. Ask him if he's like to live in a small closet where he also has to defecate and live in it until it kills him... not too nice but he might get the picture (although I won't hold my breath). Very few fish fit in a 10g and even fewer in a 5 gallon. He will obviously do what he wants which is quite sad for the poor fish.
I've cautioned him against just getting whatever will fit, but I think he's just gonna have to learn. I said he should just get two guppies or something small but I don't know if he's gonna listen or not :/