This is the age old question "when forced to, do you pick your best friend or your girlfriend?" Naturally that's a bit oversimplifying this, as it's not JUST a friend we're dealing with here.

You say they've had a past and that you know that she's still desperately in love with him. This would make me pretty uneasy so I can understand what you're talking about.

Your Trust in Him

The thing is, though, this is a matter of trust. Do you trust him that no matter how much she tries he'll always try to just be friends with her?

You Shouldn't Forbid Him

Forbidding him to see her if you ever move in together though would be a bad idea. I know that in relationships both parties have to make sacrifices and compromises but one thing that someone should never have to do is stop seeing their friends.

You Have No Problem

If he believes they are just friends then you have no problem there.