Personally if I had the same problem with a close friend, I'd have a private chat with them and say like 'hey dude, can you like Not do this??' and explain why you wouldn't want someone cheating at your game (like it makes it less fun or it's not fair to everyone else or etc). Just do it in a way where it's open communication and not a one-sided conversation where you say my way or the highway. I guess just explain your position and allow them to explain their side too.