Yes, of course people in a relationship should respect each other's wishes, assuming those wishes are reasonable and don't infringe upon the basic personhood of either of the people. I've had people I was involved with tell me what I could and couldn't do based on things that they did or didn't like when it was none of their business and didn't affect them in any way, but they thought they had the right to dictate what went on in my life. I'm sharing with you. You don't own me.
And this fellow does not own this girl. He says, point-blank, repeatedly, that he believes that controlling her is appropriate "occasionally". It's never appropriate in an adult relationship. You have to limit and control the actions of a child who is under your care and supervision, yes, but your equal and your partner? Never. It is never, ever about control. And shouldn't be. You can control yourself and your own actions. You can set limits and boundaries (as Monique pointed out) and say, "I'm not comfortable with this kind of behaviour or these circumstances". You talk about it. You sort through it. You do not ever tell your partner, "I forbid you" or "You are not allowed to". And I sincerely doubt that Evo would stand for that for three seconds.
As far as we've heard, the girl has developed a friendship with a fellow who sounds -- to me -- as if he's playing the "wounded puppy" card that that so many guys like to play on young women. Or maybe the guy's sincere and actually is a wounded puppy, although in conversation with the OP, he said something about the OP "winning" so, it sounds like a player situation to me. We're not getting the whole story, but gads I've heard and lived this story before or something awfully close to it.
So the OP fears his girlfriend may get sucked in. She very well may. She may get sucked into that, "Oh, nobody understands this poor lost soul but me!" scenario in which she further believes that she'll be the one and only person to show him how truly worthy of love he is. Yeah, yeah, yeah. And then she'll get used, and squashed, and dumped on her head. There's nothing anyone can do to prevent that. You can point it out to her, but if she doesn't want to listen, she won't.
If she's committed to her relationship, then no one else can interfere with that, even someone who has amorous intent for her. But, you see, you can't go around cutting everyone out of your life who your partner doesn't like or feel comfortable with, because controlling people are awfully, awfully good at isolating you. You have to maintain the integrity of your own person -- and that goes for both people involved in here -- and know where your limits are. If the OP's limits are that he can't tolerate her relationship with the fellow, then he has to make that plain. He can't demand or limit or "allow" or "disallow" anything. That's not his right. He can make decisions for himself based on his own boundaries. And she has to make decisions for herself based on her own boundaries.
Again, if you're doing something that makes your partner feel uncomfortable, then yes, you should sit down and honestly think about what you're doing and whether or not it's intrinsic to who you are and whether or not it realistically has anything to do with the other person. (Example: one of my exes was very uncomfortable with me wearing high-heeled shoes and dressing nicely. [Note: he was almost six inches taller than me, so that wasn't the issue.] He just didn't like me looking nice in the eyes of other people. I decided that was none of his business. Guess how long that lasted?)
And he has to set his boundaries too. If he can't live with what's going on, then talk about it. If that doesn't resolve the issue, then he has to make some decisions. However, no, "controlling" isn't ever part of the equation in a fair, mutual, adult relationship. Ever.
