Most Helpful Guy
Don't try to exaggerate how good your life is without him. Don't try to make him jealous or tell others at this get together unflattering things about him. Don't confront him and make a scene. Be yourself, be confident and focus on having a good time. Talk to your friends like you normally would if he wasn't there. I know it might be hard to act normal if there are unresolved feelings, but do your best. There will always be some type of nerves, awkwardness, it's normal around exes unless you've truly moved on. Worse case, if it gets awkward or weird, have a signal with a friend so that if you start feeling overwhelmed, they will walk away from the circle with you to give you time to calm your nerves.
Most Helpful Girl
Just try to have a good time don't acknowledge him if you don't want to