Forum:A rather simple question

ok ive been looking to an answer to what seems to be a rather straight forward question and it probably is but i havent found an answer yet.

Lets say i have a light monster with 100 atk and my freind has a monster with 3000 atk and my oponet atacks with his monster i use honest so at the damage step my monster gains his monsters atk. my question is which monster is destroyed. i belive my monster wins because my monster now has 3100 atk so my freind takes 100 damage and his monster is destroyed. he on the other hand belives that since his monster would have originaly destroyed my monster he takes the 100 damage and then my monster is destroyed.