You said it was impossible for your friend to use the mens. It clearly isn't as other transwomen do, including those who are very clearly wearing women's clothes, nails, makeup, and looking very groomed as Fionne is in that photo. Now you're saying it's because he's not happy in the men's? I expect lots of men aren't happy in the men's, they're still men just like your friend is so they use the men's.
So it's not about Fionne being happy, it's that Fionne's not a dick who thinks respecting women is unnecessary if it inconveniences a man.