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LW should NOT enter into any marriage with this guy thinking 'well, the girl will be out of the way soon enough'
Obviously the parenting will not end at age 18, nor shoud it in this case. Clearly the parents did little or nothing to prepare this kid for living. I speak from experience - all my kids, ages 6 - 11 can make their own breakfast or lunch and they can $#@)(U#@ing dress themselves. Unless LW is willing to take on a decade long commitment to helping out kid, she should keep relationship with BF informal and not contractual.
I've seen other peeps try to move into a relationship with teenage daughter, and it's not good. The kid needs a full time parent, not one parent who is waiting for kid to leave.