I know this is related to a previous thread...
I'm of the opinion that the mother of this girl is only trying to get back and the man who "robbed the cradle".
I see nothing wrong with the 18 year old being with her former teacher. If they are both happen and there is no abuse, no drugs, no etc (negative aspects of the relationship) then good for them for finding happiness.
I would tell my kid, "look, if it gets too bad, come back home. I'll have your room ready for you".