Soo... lets pretend...?

I think the best thing for you to do is to talk to Layla about all of this. see want she says. You said you and your friend are in college so im sure she is mature enough not to care seeing that she had other chances to get back with him. She has also moved on. I know it may be a uncomfortable convo but the best thing to do is ask her if she would mind. My guess is she wouldnt but you never know. If she says she would rather you not ( and for a good reason not just because she would feel aquard around him if you two all hung out bc that could easily been resolved by not talking her around him) and you value the friendship then I would probably stay away. If she says she dont mind then go for it! I wouldnt worry about your x and your crush being Best friends because that should be a matter taken up between them to. If your afraide you may feel aquard around your x then try to not be around when he is with you hopefully new boyfriend. I hope this helped and good luck!