I’d say that having it as a project is fine, build up an album of your best shots and then move on to the next. If it’s all he’s photographing then that’s not a project, it’s a fetish.
It is without consent and an invasion of privacy at the very least.
Consent isn’t required (though it’s polite to ask), and he’s presumably photographing people as they normally appear going about their business in public so there’s no invasion of privacy. If he’s snooping over backyard walls or taking surreptitious up-skirt shots then yes, we have a problem.
It’s fine to take a photo which has people in the background without their permission as they’re not the subject of the shot.Totally wrong to take a photo of anyone without their permission, where the person is the subject of the Shot.
Morally or legally? It’s not wrong as far as the law is concerned (just pick up a tabloid newspaper if you don’t believe me).
Morally, if it were me (as a rubbish amateur photographer) I’d seek permission if someone was the subject and was personally identifiable, even if it was after the shot had been taken (ie, “do you want me to delete this?”) Not that it’s a position I’ve ever put myself in.