My own dogs aren’t allowed on my furniture or upstairs.
And I have physically removed a friend’s dog from my couch on occasion. Admittedly he wasn’t supposed to be there either so it wasn’t a case of owner allowing it, but dogs can be chancers and this one was, so he was bodily removed by me. I would do the same to someone’s dog who is allowed on the furniture at home without hesitation.
And for people saying it would confuse the dog, not for long it won’t. Dogs understand boundaries perfectly well, and many dogs will do things in one setting which they don’t in another simply because they are taught the boundaries.
One of my dogs used to sleep on the chair in my mum’s house when she looked after her. She was never caught, the chair just used to be warm when my mum came downstairs, but she didn’t actually deter her. She wouldn’t have dreamt of doing it in my house. Ever.
Friend’s dog jumps on people, I caught her in mid air and put her back on the floor, within half an hour she was still jumping on anyone but me, and never has since.
You can enforce boundaries in your own house. If the owner doesn’t like it they’re free to leave the dog at home.
I had an acquaintance to stay and they insisted their dog sleep upstairs with them even though I made it clear that dogs sleep downstairs in my hous. They just took her up there. They were never invited back. There were other reasons, but this was one of the deciding factors.