Boyfriend problem!

My boyfriend has a friend he's really close with, and she is also his flatmate. He's introduced me to her and she makes sure to make me feel included in social events and stuff and also go on double dates with her. My boyfriend and her go food shopping together, cook together and go to each other's room and my boyfriend told me he has let her sleep in his room before, and he has also slept in her room. He was really open and honest about it and wasn't trying to hide anything. I told him I found that weird that they were sleeping in each others rooms, and he said he hears me, and was glad I brought it up. He said he would keep that in mind and that he let her sleep in his room because she was really sleep deprived once and that the other time his room was getting cleaned so he went to hers. I said I was fine with them going into each other's rooms but not sleeping in each other's beds, and he acknowledged that. Should I have made him give his word on not to do something like that again no matter the circumstance? I felt like I was being not that firm when talking about it.