Are his fish 15", or would they be in the immediate future? Unlikely.
Saying he cant have three crappie in a sixty because they have the potential to reach 15 is like saying I cant put my gatf in the 510 gallon, eight foor tank because the fish can get six feet long. Both myself and the OP have a very long time before the fish would need a bigger home. By the time his baby crappie are big enough to need moved, he'll be old enough to get a summer job and buy a 125 off of craigslist. By the time theyre 15" (if they even do, just be ause some do doesnt mean every one will) he'll be out of high school and probably living on his own.
OP, buy two more crappie and a good filter and go enjoy your fish. Your tank will be fine with three crappie, a sunnie and a pikerel for a long time.