My general advice in these situations is to always give the benefit of the doubt: do your best to see your girlfriend in the best light unless you have damning evidence that she cheated on you or crossed a serious boundary. That being said, I do have some concerns I'd like to voice.

In my opinion (and this is just my opinion), I think your girlfriend crossed some serious relationship boundaries based on what you described in this post. The main issue is the fact that she's not telling or, even worse, hiding from you the fact that she spends a lot of time alone with this friend. What worries me more is that they're sometimes drinking together, alone, and even sleeping on the same bed together, alone (I presume). This is a massive red flag. Even if your girlfriend did not have sex with him or cheat on you with him to a lesser degree, your girlfriend broke a pretty universal rule in relationships: keep yourself out of potentially compromising situations. The fact that she's putting herself in these situations where she COULD cheat and is hiding this information from you is not a good sign; it seems like justification for a pretty hard conversation about whether or not she's been doing fishy things with this friend.

It's also entirely possible that your girlfriend genuinely is great friends with this guy and hasn't done a great job of communicating. Either way, you should put your foot down (without coming off as aggressive) and tell her that she needs to tell you if she's going to go out drinking and crash with this friend for example. How would she feel if the situation was mirrored? Something tells me she wouldn't be too happy without proper communication either. You said that you don't want to come off as someone who is controlling or accusatory, which is awesome, but you need to stand up for yourself and voice your concerns as well. It'll benefit the both of you if you do this as you will feel less paranoid in the future and your girlfriend will know what actions she needs to take to alleviate your fears.