first you've got to figure out how much he means to you.
if you think you could be going somewhere with the relationship talk to her and then pursue it.
but if you don't like him that much and it's just going to hurt her, don't do it. i suggest you tell him that the both of you should wait. if he's just got out of a relationship, no matter what he's not in the state to start another one. even if he's not hurting, rebound relationships are 99% of the time a bad idea.