FYI, this means that you need to catch him in the act of urinating inside and interrupt him with a sharp "NO", then lead him outside. Dogs don't respond to retrograde punishment and trying the "rub his nose in it" crap is only going to make him feel afraid of humans in general, which will lead to more inappropriate urination. Nasty cycle.

It wont be the first time a dog pees in his house if so. But most likely if the dogs get along he will acknowledge that it is your friends dogs territory and y'all are visitors. So he shouldn't unless he has to go bad enough and no one will let him out.

I have a lot of experience with this kind of thing.
Best idea is to have the two dogs meet eachother some where other than "home" for either. For example have then meet at a park first. That way there isn't any territorial bias in the first meeting from either of them. If they do well together at a park, then you should be fine bringing your dog to your friends home.

Hope I helped! Have a good day