I want to date my ex's roommate/good friend?

Me and my ex girlfriend dated for two months and broke up in april. She does not want to be on speaking terms with me, but all of her friends still like me and understand why we broke up. The thing is, one of her good friends (who is also her roommate and part of her squad) has a crush on me, and i on her. Everyone knows it. She's leaving to go home to europe in two months, and i'll never see her again after january. While I don't want a long term relationship, it'd still be nice to have a casual thing while it lasts.

Thing is, I don't know what boundaries I'm crossing here. I do feel like scum for hitting on my ex's best friend, and I don't wanna come between them, but i've heard that my ex has been blowing up tinder, and has moved on (mostly) from april. I really like this girl, and don't wanna let this oppurtunity go to waste. Hit me with your wisdom Y! Answers, should I do it?