I think that since you are helping your friend out and opening your house to her and her dog, they need to follow the same rules your dogs do. My dogs are allowed on the furniture at home, but not at my MIL's house. They very sledom even try...they have learned, when at grandma's stay off the couch.

Ours are also aloud to sleep on our bed most nights. Last night, I trimmed the dogs nails, and fresh cut nails hurt We told the dogs they were sleeping onthe floor, and they did, no issues. They were not being punished, or treatly unfairly, but we are alpha and make the rules. In your house you have the right to say the same thing.

First make sure your friend will help you enforce the no furniture rule, then when she tries to get on the furniture, tell her "no" ( or "get down", or whatever command you want to use), or and give her a gentle nudge down. Praise and reward her for getting (and staying down).