First off, this speaks volumes for the type of person you are to be included.
Being divorced, I've done several things with the Ex's family.
I've done several night outs with the ex bil and sil.
You mentioned you were friends with your x's dad. How well do you get along with the mom etc.
Would you have a good time going? You mentioned you didn't think, the x bf would be there. What happens if he is there? Could you still have a good time?
Lastly, who invited you? Is there a hidden motive there to get you back with their son?
Do you see a remote chance of that if he's sober etc. ?