I know a couple of folks who transitioned thusly, retained their trans friends and continue to hang out in the community. For one, I was present during one's "de"-transition (it's just a transition). It went fine, because the community I know is supportive of how folks identify their gender. They talked about not feeling right for a few months, later announced that their pronouns were changing, so we supported them when they felt off and changed pronouns when asked. It was a fairly simple matter, really. It's not uncommon for trans / nonbinary people to experience fluidity over short or long time periods, and "de"-transition is on that spectrum. No biggie, the world keeps turning and we're there for eachother.

Notably, I've heard one such person say something along the lines of "well, no, I'm not cisgender, I transitioned twice so I'm definitely trans." Labels are pretty silly that way.

>the nature of the gp's claim would be impossible to prove or disprove without mind reading powers

Or by using inferential statistics.