there's 2 issues here.
The first is the 1-2-3 is insecure. He obviously thinks that his GF is capable of cheating - I don't know him, so I don't know if that is because he's that kind of person (insecure, jealous) or because she's actually cheating on him.
Either way, he's got to move on.
If he's insecure within his relationship, then he should break it off with her and fix it. If he tries to fix it while he's in a relationship, it's going to cause problems because emotions will run high. She's going to see his insecurity, and thats not good. He needs to be confident and secure - and he's not, so fix that, then if he still want her, get back in contact with her.
The second issue - it seems like her actions are inappropriate. Someone mentioned earlier - there are other places to study other than in his room. Perhaps Library, common room. She could invite him over when 1-2-3 is around. She shouldn't place 1-2-3 in a situation like this. Becuase more than likely her friend will be trying to get into her panties - and if the chance exists, would probably bang her. End it now, 1-2-3 retains his self dignity and respect.
I'll tell you somethign that happened to me. My ex's ex BF called her 9 years after their break up. He had just got a divorce and they had not conbtacted each other since they broke up. He calls her, tells her he recently got married and wanted to meet her for drinks. She told me, and said that it was her friend and she was going to meet him. My response. How can he be your friend - he hasn't spoken, or called you in 9 yrs. He's divorced and going through his black book looking for some poon - thats why he calling you. She said -= No, he was married. He didn't call me out of respect for his wife. I said - then why arre you going to meet him - is that because you don't respect me? - if you meet him it's over. I leave. I'm not going to be placed in that situation.
She didn't go.
She agreed , that he probably would fvck her again, but said that's something she would not do - would not even think about. But, that she understands where I'm coming from - and she didn't go.
He should dump her. fix any problems he has - or move on if she's playing him