She’s not ‘working shifts’ or ‘weird hours’, she’s working evenings. Staying up for a bit when she gets home and getting up later is fine. Unnecessarily staying up all night and sleeping all day is not. It’s not healthy sleeping all daylight hours - those who have to work nights, have to, but it’s not good for you. It’s NOT being a ‘sleep snob’ it’s a fact.
It sounds, to me, like she’s doing it to avoid the rest of the family, like she’s using the place like a free Hotel and that would piss etc me off. If you dislike your family that much, move out.
If it’s to ‘game’ then that’s not healthy either. Hours and hours everynight is not healthy. Yes, technically, she’s an adult, but barely, she’s 18 and I wouldn’t just ignore her gaming like that.
Your DH is being utterly fucking ridiculous however. You can’t not go on days out because you’re 18yo doesn’t want to go. FFS.
I would tell her the above and say she has until the kids school term starts to do this if she must, but then it’s back to normal and she’s up and about at a reasonable hour, not living like some nocturnal animal.