I'll concur with everyone else here regarding self denial and how incredibly strong it can be. About a year ago I saw that news story that was floating around about the couple (mtf and ftm), and I literally said out loud that I was jealous because the mtf gets to live the rest of her life as a girl and be seen that way by everyone and be accepted.
It took a few more months before I realized I was trans. The framing of "transgender" and whatever was going on in my head were just two completely different things, I was something but it wasn't trans? I don't even know. Internalized transphobia and other things, I guess.
Anyways, my advice is to make it okay for them to explore. Perhaps even ever-so-gently encourage that exploration, in any form, wherever they may show an interest. If they're willing to open themselves up and explore the scary things then things may begin to change, perhaps rapidly, perhaps slowly. But they may not be ready for that, or feel they're in a secure enough position (re: job, other family, friends, etc) to be so vulnerable. Gotta man up for those counting on you, don't you know!
It took me about a year of this sort of self-exploration to get to the self-diagnosis of transgender, with lots of time to process between exploratory missions.
Good luck, I hope things work out well and you two find your happiness
