It seems that you're upset because she didn't do what you wanted her to. However, she's an adult and is capable of making her own decisions.
Her actions did not harm you in any way;
(unknown) they just made you uncomfortable and jealous. But you nevertheless want to be able to tell her what she can and cannot do in her own life, and then browbeat her and make her feel guilty for making her own choices irrespective of what YOU think she should be doing.
That is pretty much the definition of controlling. My guess it is an issue for most couples when one of them hangs out in a hotel room with someone who is sexually interested in them. The OP could be controlling or it could be that a boundary has been crossed.
And if she wants to cheat on you, there is absolutely nothing you can—or should—do to stop her. Unless I missed it, the cheating aspect is unknown, but until it is verified, he should voice his unhappiness and not bottle it up. She is not your property, and wouldn't be even if the two of you were married. If there are boundaries she knows not to cross if she wants to stay in a relationship with you, and you trust her enough to not cross those boundaries, then you owe her the respect to make her own decisions without feeling like you have to check up on her, or tell her what's appropriate and what isn't. It sounds like a boundary has been crossed and his girlfriend has made it clear hanging with this person is more important than any discomfort it may be causing the OP. The problem is that you DON'T trust her, or you would have nothing to worry about. Trust is based partially on actions. And your pretense of "caring" about her not putting herself in danger is unconvincing. Be honest with yourself—
you know there was zero chance of her getting raped or harmed. Quite the crystal ball. 
The guy may be a sleaze, but it's a long way from a sleaze to a rapist. The only thing you're communicating with your "concern" is that you think your girlfriend is a moron who can't tell when she's putting herself in danger.
If you really did trust and respect her, this whole thing would be a nonissue for both of you. Based on the OP's original description this would be an issue for most couples. If you don't believe that, ask yourself why she did it. She clearly knew you weren't okay with it, and did it anyway. It sounds like she was bristling at your controlling nature and the fact that you fundamentally didn't trust her or her judgement, and thus decided to assert her independence. I don't blame her a bit. If somebody tried to tell me that I wasn't smart, mature, or sensible enough to handle a situation I was voluntarily putting myself into, my response would pretty much be, "Screw you; I'm a grown-up and I can handle myself." I would have done the same thing she did in her position, except I would have told you to back off and not been apologetic about it later.
If you want my advice, go to her and tell her you're sorry for making it an issue, and that you really do trust her and her judgement. Tell her you respect her as an adult, and that you were wrong to get so upset about her making her own decisions about what was in her best interest. Then cool it with the whole jealousy thing. Seriously. It will only drive you apart. Do you honestly think if, when she said she was going to go to the guy's hotel room, you had just said "okay, have a good time", that she would have cheated on you? If so, then ask yourself if you can really stay in a relationship with a woman who is inclined to cheat on you as soon as you stop watching her like a hawk. And if not, why did you feel the need to make a big deal over nothing?
Think it over for a bit. You can either continue to punish her and make her feel guilty for doing nothing at all wrong except not doing what you wanted her to, or you can use the opportunity to build trust and respect in your relationship. It sounds like right now, there isn't very much trust or respect between the two of you. You can go and blame her for that if you like, but it's really the fault of both of you. Respect begets respect, and trust begets trust. Start showing her some, and you might gain a partner for life. But if you'd rather have an obedient pet, get a dog.
