I spend time with women I'm friends with and my girlfriend spends time with guys she's friends with, so I don't think that alone is a big deal provided there's good communication and both parties are comfortable with it.

However, you've expressed your discomfort and she doesn't seem to think that's important. She's not making your needs a priority in her life, by the sounds of it. Coupled with the other signs of growing distance between you, I don't think you're wrong to be concerned about her hanging out with this dude. Unfortunately, it doesn't sound like she's going to change those plans or do anything to acknowledge that she's pushing you away, so she's effectively told you that she is moving on even if she hasn't said the words out loud.

I'm usually the last one to go straight to "dump them!" on people's posts, but in this case it sounds like the relationship is already over, you guys just haven't acknowledged it yet. I'd sit her down tonight and tell her that given the obvious distance between you two - and the fact that this distance doesn't seem to bother her or constitute a problem - that you think it's best if you go your separate ways. It's better to just move on with your lives than draw this out anyway, and if you end it now you'll at least have the comfort of knowing that you got out before she cheated, and ended it on your terms. You've gotta stand up for your own boundaries here, and treat yourself with respect because she isn't going to.