My boyfriend and I share the same best friend and for that reason it caused a lot of issues because I felt like she always sided with him & pretended to be "there" for me at the same time. When I told my boyfriend we can't be together, 2 days later, my best friend drove to his place, partied with him (& the rest of our friends- I wasn't there out of my own choice) and slept over his apartment. Who knows if anything happened. He can do what he wants, but why did she not consider my feelings?

Well this is the problem when couples have the same friends. When things go south people are forced to choose, even if they don't have to. She might have chosen him over you. As well I know that you're grieving your relationship, but it still isn't all about you. She should be respectful of your feelings, but they're not going to be top most in her mind.

She's your bestie, I understand that, but she's got her life to live too. She went to the party, might have gotten too drunk to go home and stayed over until she's sober enough to drive. There are a bunch of reasons she stayed the night. I wouldn't think too much on it to be honest. You won't be able to dictate what your friend does.

I guess you can't consider her feelings either, as hurt as you are after this break up. Is it fair to even make a good friend to you both, choose sides after you break up?

The way I see it it's time to put this whole triangle behind you since she wasn't on your side during your disputes, and honestly never should have been put into that position in the first place. Maye when the dust settles on your feelings you rethink this best friend and make better choices on whom you consider a best friend. Obviously not a good idea to share best friends with a boyfriend in the first place.

That was asking for trouble.

In some ways you can't expect her to avoid him - after all, he's your EX.
Plus other people were there! For all you know, she was there just because of them.
Maybe you hoped she would mope around with you and commiserate, and that would be nice, but she didn't.
Your title question is 'Was it OK.' There's no real right or wrong. It's your choice based on how you feel about how she acts around you after the break up. She's the one in the unique position of being the only one connected to each of you.
Personally I would retreat. If she cares about you enough, she will see that she is losing you and make an effort.
If she doesn't, so be it.

If she slept over there because she was drunk and didn't want to drive, that is a good decision on her part and for everyone else that would have been on the road that night. However, if that was the case she should have come forward with that to assure you nothing happened. He as well. Additionally me thinks you are using the term "best friend" too loosely. I have a couple of bestfriends and some very close friends, and none of them would ever come close to behaving inappropriate or not assuring me there was nothing inappropriate.

Agree with Tal - rethink this whole situation.

He as well, Oliver? He's an ex.

Ohhhhhh. Didn't see that in the title. Gracias. Well then they are free to do what they want to do and the OP doesn't get a vote. It might not be the nicest thing to do to a best friend but still an ex is an ex so he's fair game.

He is an ex, and guess what, sometimes, they may start dating your friends. I am not saying this is going to happen, but it does.

If you are over with him, and do not care about him, does it really matter, if your friend still likes him?

My ex, is now dating my best friend, I do understand it can be weird, but they each have their own lives.