Hi Mom. This was a little confusing (or maybe I'm just tired). But here goes.
Your friend doesn't have enough boundaries with her dad. He doesn't seem to have anything to do but hang out at his daughter's place. He also doesn't seem to understand how old she is and that she has a life of her own that doesn't have to always include him.
It is her job to establish boundaries with him. Instead of getting into an argument about it, she just needs to tell him "No, dad, not this weekend. I'm not available." And then she needs to lock the door and leave. If he has a key to her place, she needs to change the locks.
This is part of your friend's fault, Mom. She has allowed her dad to do this stuff to her.
It would be easier if she had a computer to send him written emails so he couldn't pretend she didn't tell him, but without it, she is just going to have to turn him away at the door if he shows up, or not answer the door. Perhaps she could take her car out of the driveway so that he doesn't think she is home.
At her age, Mom, she needs to take the emotion out of this and just say "Dad, I'm not available. Please find something else to do this weekend." And then hang up the phone. If she refuses to argue with him, there can't be a fight about it.
Tell her good luck from us...
Dawn