Hello and welcome
So, I'm assuming he was fully housebroken and didn't do this where you lived previously? I'd say you just have to go back to basics of housebreaking as a little refresher course for him if that's the case. Could be he's stressed from the move or just isn't sure about the new house and what he's supposed to do there. Are there other pets there at all? Is he nuetered?? That's a big one cause if not, it would be very natural for him to territory mark a new home.
You'll just have to watch him very closely and follow him around the house so you can catch him in the act and quickly take him outside. Like I said, if he's previously housebroken, it should take long for him to get in the swing of things at your friend's house.