My exH and his GF have moved into a new house together and GF told my son (20) that the room in the basement was his. He could put whatever pictures he wants on the walls and leave clothes, games whatever there. It will also be a guest room but he will always have first dibs.

He came home and told me all of this in a snide mocking tone of voice. He said isn't that crazy, I have a home here? He is leaving for school in 4 days and is only back for Xmas, Reading week and 4 months of summer holidays.

I said no, it's not crazy. It's actually very nice of her. She's trying to make him feel comfortable in his Dad's new home, she's trying to let him know that he has a place there too if he wants it.
He hadn't seen it that way and I think he was trying to prove to me that he is loyal to me even thought his Dad has a new woman in his life. This is the first woman exH has introduced to him in the 15 plus years we've been apart. He did date women but never introduced them to our kids.

I think it's important as BMs to remember that our kids take their cues from us. If we don't like the SM then they won't, if we think they are overstepping bounds then so will the kids.
Even if all those things are true, it's something to be taken up with the exH, not talked about to the kids.

I wish there was a way to make all BMs realize that their kids will love them no matter what. That it isn't a competition and that the more people who love and care about your kids the safer and happier they will be.