I would have an open and honest conversation about this topic. Tell your kid that you are proud of their success and transition to a new phase in life. Tell them that it’s a great time to meet people especially during the first semester when people are all in a new situation. Tell them you want them to have the whole experience of college life. Also tell them that you don’t want X to spend time in their room unless she also intends to be a student there. I would definitely get the point across. I’d also check the policies to see how much the BFF is allowed to stay. Hopefully, it’s not a lot.
My kid had a BFF who was really an egomaniac. Had to be the center of attention. We told our kid we thought this person added little value to the friendship. We never spoke badly about the BFF. But we also didn’t support the friendship. BFF moved on. Last we heard the BFF had someone else’s parents driving her to college campus. BFF was an international so needed a lot of support and we could see the red flags miles away. Lots of demands in the form of requests and no graciousness or thanks. Flared up a lot during Covid. No thanks.
We also make an effort to celebrate our kids other friendships and talk about how much we liked X, Y and Z and how nice they seemed. Our kid is an adult now. But even if kids don’t seem to be listening they are. They know if they are being used by a BFF or if a BFF isn’t kind etc. This BFF could block your kid establishing roots at college and that would not be ideal.