Hey dude, I'm Muslim and currently in my own mess of confusion with the whole Islam thing and it seems to me that the stance is at best, vague and pretty situational. I've sorta come out to my parents in the last month and they're more concerned than anything else, though I haven't explicitly told them that I wanted to transition. Islam's point of view on the matter is very important to both myself and my parents, and I'm pretty sure that if I'm convinced that the answer is no then I won't do it. I've spent much time, as have my parents, trying to find an answer and I've personally found mixed responses, while my parents have found ones that say it isn't allowed, though they're still open to getting other opinions. That's where I'm at right now.
It would depend a lot on how religious her family is, because it's not like all Muslim families are very conservative, though mine tend to lean much more in that direction. Pretty sure some would be okay with it, and I doubt anyone would go as far as disown her for something she isn't even doing herself. I mean, the fact that she's dating someone means her family as it least sort of liberal? Assuming they know she's dating you of course. I think since you transitioned so young, it shouldn't be a problem at all, even if you asked a religious scholar. But I think if it comes to that, she could always consult a scholar on the matter and use that as back up in case her parents aren't so accepting. I'm no scholar, and honestly there are way too many complexities and nuances and I haven't a clue about, but at least in my perspective Islam would say, what's done is done and you can't really go "back to being a woman" and hence should be treated as a guy? Again, you'd need a scholar to give you proper information, this is just what I'd imagine they'd say. I think I've read somewhere that if someone who transitioned as MtF and died female they should be treated as a biological female when it comes to all the funeral arrangements.
But yeah, by all means message me if there's anything you wanna discuss. I'd be happy to talk.