Dig the fucking tunnel.
Regardless, I consider anyone operating in humanity to be mentally ill in general. It’s just, how much of your mental illness impacts the rest of us, and how much of it is abstracted away so we can quickly label it harmless (mostly to the rest of us).
Digging a tunnel the closest shit to “ain’t bothering me” as it’s going to get.
Now, if your nonsense pushes you to monopolize industries and abuse labor for profit, yeah, now your mental illness is on blast.
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Few would consider that the GF is mentally ill, so obsessed with how it would look to others, so obsessed with how it’s non conforming.
If they're in a rural area where those aren't potential issues and the boyfriend doesn't want to change, then the partner simply has to decide if they can live with the new situation. I think we'd agree on that.
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I'm confused on your comments about reading into the partner's mental health. I see no indication in their post saying they're concerned about how it looks to others. As for non-conforming, they seem to be somewhat accepting, stating that they haven't made it a big issue before because the boyfriend seems relatively happy. They specifically say, "My biggest concern is his safety". That seems reasonable. Major behavioral changes on the other hand would be a bigger warning sign. (while acknowledging that a red flag doesn't mean there's an actual issue)
EDIT: From an update [1] here's why the boyfriend says he does it: "It’s just pleasant. When I’m down there, I feel safe and calm, and I’m always happier when I leave than when I went in"
I suppose there are worse coping mechanisms than that.
[1] https://www.reddit.com/r/BestofRedditorUpdates/comments/u6a3...