I want to build them both up, and make them both happy. I want to love her because obviously, she DOES deserve love! BECAUSE HE LOVES HER!
I like that philosophy.
Wouldn't it be nice to have such a generous spirit that all pettiness and resentment passes beneath you like ripples under a sailboat? I suppose in terms of the metaphor you would still respond to the urgency of joy and love just like a sailboat responds to the wind, but the small stuff -- small-minded
stuff -- would just slide past and away from you.
I dunno if that's possible for anyone but the Dalai Lama, but it's nice for me to imagine.
On a practical level it sounds like you're already doing good things -- asking about her, making thoughtful gestures toward her. My GF's husband and I do similar things -- we're not close, but we're aware of and considerate toward each other, and we can welcome each other with honest affection when we do meet. Maybe that's where it could
come down with you and your BF's (BF-in-potentia?) partner.
"Could" because if she really is controlling and is also centered on him as the only relationship that matters to her -- you write that she has no other friends, and alienates her potential friends -- then that sounds like a dynamic which cannot be overcome from the outside. You cannot heal whatever damaged her psyche. She has to do that.
Even when a couple's relationship is really dysfunctional I tend to think it's counterproductive for a third party to try to fix things. Just like with a damaged individual, an outsider cannot heal whatever's broken in there.
But it's a tough line to walk, isn't it? When someone is complaining about the behavior of a relationship partner I tend to waffle and say things like, "Well, she must feel really bad about herself if she's acting that way to you." But I suspect you know that technique already, you seem conscious about that kind of stuff.
I wish he did not have to be less than honest and transparent in this, though. I understand his difficulty, but it makes me nervous that he can't act in a plain, forthright way at this point.
Good luck! You may have to be just a good friend to him until his primary relationship either relaxes and opens, or disintegrates. Either way, if you've been a kind and generous friend you will be on his short list of best lovers if that becomes possible.