After checking the rules it's my understanding that all figures(friendly or monster) block line of sight. Some of my friends are arguing only monsters(not other heroes) block the heroes line sight, but i cannot find this stated anywhere. Thanks

LOS is pp.9-10 in the rulebook. Reading these pages should answer your question. If not, ask again.

heroes as well as monsters block los. but keep in mind that it´s not allowed to spawn new monsters behind other monsters. in that case they´re not blocking your los.