A friend of mine invited herself to the beach house I share with my boyfriend. I don't want her to come. A co-worker of mine keeps asking me inappropriate questions about my family. They are all related to boundary issues. Without the answers to these questions, friendships can be broken, work relationships may suffer, and family relationships may become strained.
Honest communication is the key to creating a healthy boundary.
If Dan doesn't communicate with her openly about his discomfort from the outset, the situation will likely get significantly worse. She will unknowingly move in, his discomfort will turn into resentment and their relationship will either become antagonistic or icily distant. What should have been a simple healthy boundary became a barrier instead. Boundaries become barriers when communication closes down. The longer she stays, the more impossible it will seem to be for Dan to broach the topic and the angrier he will get inside.