The letter writer mentions her (ex-)boyfriend's OK Cupid account screenname in the comments. I looked at it and didn't recognize him. I checked the same screenname on Reddit, which she said he also used (no account under that name) and here (an account exists by that name, but I don't think it's the same person - in particular the OKC account has a characteristic punctuation error that the local account doesn't make). If anyone from Missouri wants to see if he looks familiar there are breadcrumbs to follow.

It's possible that the choice of the word "rationalist" was a coincidence and this is not a peripheral community member mistreating his Muggle girlfriend, but just some random guy. I think it is worth finding out if we can.