How should I act when I see my ex?

We ended on bad terms (his fault, not mine). And I haven't seen him around campus ever since. But this club that I'm in is going to an event this Friday and he's going with his club which means I'll be seeing him for the first time in ages.

I see his friends all the time and they always come up to me and give me hugs and stuff. My friends hate him for what he did. He pulled a d*** move, but I got over it. It's just that he has a girlfriend and I don't know if she'll be there, but that'll be pretty awkward.

What should I do when I see him? Flash a smile? Go say hi? See if he says hi first? We used to be best friends. I don't want to pretend I don't know him...
How should I act when I see my ex?
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