I really wish my girlfriend wouldn't be friends with this guy who has asked her for a threesome multiple times.
I can see why that would make you uncomfortable. How did you girlfriend react to his proposition?
He's asked her multiple times to meet up and she's never accepted. She thinks he's a nice man. I've known him for five years and he's my friend and his wife is my wife's friend.
It's good that you want to set boundaries with your wife. Generally, it's not a good idea to tell your spouse what to do, but if they're doing something that puts your relationship in jeopardy then it's important to speak up. Hanging out with a man who's asking for a threesome isn't a good idea, and it's reasonable to be upset with your wife if she continues to spend time with him.
I'm hoping she won't get too upset when I talk about my concerns.
I think she will understand where you are coming from if you explain that it makes you uncomfortable that she is friends with someone who regularly asks her to be in a threesome. It's only fair though that you stop being friends with him and his wife as well though. If you are going to ask her to stop talking to him, then you need to do the same.