I'm not sure if it's a medical issue, but it definitely seems like a bigger safety concern than the nudism. (I suspect it's not a mental illness, just a douche who thinks he's too original and special and enlightened to abide by conventional restrictions like not playing a ukulele while driving, or not exposing yourself to people who didn't agree to that in advance.)

Also she wonders if she's crazy, he's apologised but not changed his behaviour, she's asked him to go home to his mother... that does not sound like a relationship that should be continuing.