@Dashinka nailed it.
Whatever your stepdaughter does, she must NOT get married before she enters the US and activates her LPR status.
As an aside, 18/20 years old is still pretty young and a lot can change. Best for your stepdaughter to activate her green card, get used to life in the US, get a stable job of her own, and THEN if she still wants to petition the guy, she can get married and file for him herself.