Recently our 17-year-old daughter introduced her boyfriend to us over dinner. He came with another girl which we initially thought was a common friend. No, that was actually his other girlfriend. Super nice girl, very pretty. That dinner was awkward to say the least. My husband already doesn’t like him due to this whole poly thing. I just don’t understand how our daughter could be okay with this, no matter how good the guy may be.
I think the only thing you can do is remind her to be careful with her heart, and be really honest with herself about what she wants. I.e. Does she want him, or her, or does she want them both because there is twice the potential to be hurt when there are twice the number of partners.
Anything more than that will risk alienation.