I'm starting to understand the repetition rule..

http://en.lichess.org/nFb9GdsG#107

I tried to explain that it was a draw, but he refused and thought he could win... why

Well, one of you could always make a blunder that results in a win. If he wants to play through the full 50 moves then you have to play with him.

White could force a win after move 49. rf3 kc1 50. re3 kb1 51. rd3 ka1 52. rc3...

if 52... ka2 53. ra3/rb3
52... kb2 53. ra3/rb3/rc2/rc1
52... kb1 53. rc2/rc1