I don't want to upset you and I am the mother of two nice adults too. However I feel it is my duty to let you know that only your daughter when she is tired of being taken advantage of is the only one that can decide when and it she has had enough. Your daughter has chosen her and she has to decide if she is an opportunist. Every thing in life is a lesson and when things happen that are hurtful we learn from them. You can't teach her what she hasn't learned from life yet. She will decide at some point being the responsible adult if she cares to continue this relationship. You will cause more harm than good by telling your daughter more than once about her girlfriend. Once is helpful advice. More than that is a wedge between you and her.
I would instead offer advice that gets your point home without sounding like criticism. When she comes for dinner use this as a learning experience. Say very nicely something that addresses her being an imposition. When you address the problem a little at a time you may make a bigger impact
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