Yes, he's done this 3 or 4 times over the years. At the time, he feels something about the job is bad for him. Generally after a break, sometimes with counselling he also gets the perspective on it that his anxiety has created some of the tension/issues.
Then things are fine for a while, this time 3 years. A new manager just started at his work and she has an abrupt style which he finds confrontational. There are others there who are much more supportive though, so there is good with the bad. He is off on sick leave but that'd about to run out and he tells me he should decide today whether to resign or not. We both fear that, if he returns, he'll start shouting at people. That would be even worse than just leaving abruptly. It's so hard seeing him so unhappy....
I read this, which I fouind interesting:
"Sometimes people get into patterns of coping with anxiety that cause it to snowball. They overthink (ruminating about the past or worrying about the future), avoid whatever triggers their anxiety, and use compensatory strategies—like being extremely perfectionist to avoid feeling like an imposter at work—that decrease their anxiety temporarily but increase it over the long-term."