If they are on the larger side, the female should have a pink belly. That's not always 100% though. They would love a planted tank, maybe with some driftwood .
try to get a pair of German Blue Rams, Mikrogeophagus ramirezi. If he can take good enough care of them to get them to spawn and produce free swimmers you can upgrade him to a 29 or 55
I also like the idea of it being planted, but that adds more work to the spectrum than a 15 year old might have the time or interest for. just my .02
German Blues and planted is definitely the choice. My dilemna comes in finding the proper ratio, 2 females : 1 male....And how to distinguish. Prob a few cory's as well.