3/6 reraise with 7s 4s

[pipedown3]

my friend and I are at an impass over the following hand. in a loose 3 6 game 2 players limped and a third raised [ I folded Kc 10h ] my friend seated on my left reraised, 4 players called and the 2 limpers called. the initial raiser reraised and my friend again raised, capping the action. everyone called.
the flop came 7c 5s 7h.
there was plenty of action all the way to the river and no one folded.
my friend took down the enormous pot showing down 7s 4s.
he has been defending his play, which i do not fault after his initial reraise. he can't fold.
the play i cannot defend is his reraise. my position is that cold calling the initial raise is incorrect, i do not understand how reraising can be correct [which my friend claims]
of course he was the hero in this story having won the pot, but i argue that he should not have been in the hand.
any thoughts?

pipedown3