Here's the central problem: you (and most of the contributors here) are ascribing NT motivations to somebody who's ND.
These things do not mix.
Consider his position: the entire world is difficult to communicate with. Everybody he meets, he has to struggle to fit himself into a box that says "NT" on the front, to avoid being socially rejected. That's an extremely hard thing to do, and it wears people down. He has this one friendship where he doesn't need to do that, and the solution is to make him get rid of that friendship?
That's a recipe for the end of your relationship, in my opinion, because it only fixes a symptom and not the core cause. The chances are, from the ND perspective, that he doesn't even see her as "a woman", but rather "a person who needs help that he can give".
First off, make sure that you explain how you feel with direct words and not implications. From what you've written, he's not going to just pick up on things (I'm very much the same). Talk it through as unemotionally as you can, as though you're trying to fix a problem with a washing machine.
Secondly, there's a reasonably high chance that he feels a sense of obligation and responsibility to help his friend. That's something that's not necessarily going to go away, but when you know it's there...you can plan for it. You can even offer to help, which may go some way to rectifying the situation from the inside, as it were.
Last...address the central problem: he finds it easier to talk to folk with his particular flavour of ND-ness. This is not unusual, and it's a direct consequence of being forced to mask with everybody else...including you. This is explicitly not because he doesn't trust you, and you shouldn't feel insulted by it; he's simply applying the things he's learned over the years in order to make you happy and make your life easier. The solution is for you both to work to make "home" a place where he can feel comfortable without masking (one wrinkle being that he may not even be aware of his own masking, just that some things aren't necessary when talking to ND people).
That's easier said than done, though; my wife was absolutely insistent that I stop masking when we found out that I'm autistic, but wasn't quite prepared for what that actually entailed. After a couple of years, we settled into a happy "medium place". I don't mask anywhere near as heavily as I used to, but I also know there are some traits which she finds hard to tolerate or understand, so I kind of half-mask just for her sake. Basically, if you want to go down this road, then both of you need to be aware that it's going to be a bit hit-or-miss with experimentation for a while.
Of course, take all of this in the context that I obviously don't know exactly what his traits are, and they may relate to his experience in a completely different way to mine and the folk I've heard similar issues from.