Is it O.K. to date your friend's ex?

Here's the story: My friend dated this girl and they broke up like 8 months ago from today. Now he has a new girlfriend so I'm guessing he's over his ex but she's still single. I can guarantee that I never liked her when they were dating but I'm getting feelings for her now as she's in my homeroom cl. I'm pretty sure she likes me too since I heard her talking to her friend about this guy from her cl and it resembles me. Now back to my friend. We're not that distant-friends and I still talk to him occasionally but we're not tight. I've nvr been in a relationship before and I really do like her but I don't wanna screw things up with my friend. I was thinking that it might be okay since he has a new gf but I don't know. What do you guys think and what should I do?