This doesn't happen
Though
technically you're not supposed to allow anybody to watch a movie that didn't purchase it, those laws are more in place to give people that are major violators no loopholes in the rare cases that they prosecute
I will say that I have bought movies before by virtue of watching them at a friend's house and their being good. The difference between that and piracy is that:
1.)You are dependent on the friend to make the movie available (ie. it does not have unlimited convenience)
2.)When you watch a movie with friends, you're there to hang out with them, not watch that specific movie.
3.)Any time you borrow a movie, the original owner is ceding his ability to watch it so that you can
These are kinda sorta important distinctions for why watching it with a friend isn't the same as piracy
Due to points 1 and 3 you are much, much more likely to buy in this scenario than if you pirate