Does the friend know about the A?
The first couple times post-A when I would meet a friend, it would feel awkward because of this huge "thing" that I had done, and also it was weird because the friend would be BS-approved, a friend of the M, but maybe the friend didn't know anything had happened. So, there's this huge elephant in the room that only you can see.
then there is the whole "having a good time" thing...when you're off spending time with someone other than the BS, you probably generally feel like shit, or sad, or whatever, and you will feel any happiness you might experience out with this friend may either be false, or that you don't deserve to feel any happiness, especially if your BS isn't there with you to share.
I think it's generally a fake it until you make it thing. It's a step that has to be taken, but you won't know if you're ready for it until you do it.
I'd say give it a chance and see how it goes. Be normal. Try to take the experience for what it is. Have a debrief with your BH when you get home and talk about how you felt.