I don't see anything wrong with smoking a joint or bowl in the evening, same as I don't' see anything wrong with having a beer, glass of wine, or cocktail in the evening.
It's when you can't skip that intoxicant for an evening or two that there's a problem.
A lot of guys when husband was in the Army found out they had an alcohol problem when they went to the field and suddenly couldn't go to the bar every night. Not frank DTs, but a lot of shaky, sweaty, anxious folks for a few days.
To me, that's the tell, can she go without if she chooses/has to? I do know a lot of people in my age group who started smoking in the 70s and still smoke.
They have perfectly functional lives and careers. Some have families, some don't, but all have done well for themselves.
Others have not done so well.
Many, like me, quit smoking during or shortly after the 70's. Some quit all drugs, including alcohol.
I do want to mention that for me, the gateway drug was tobacco, not marijuana.
Since your daughter is 18, there is nothing you can do about her cannabis use beyond not allowing it on your property and giving her the choice of living under that restriction or living somewhere else. That's it.