Polyamory is more common these days. It may be experimentation or real for her - only she can decide that. Be there for her, try to not to seem judgemental or negative so that she can trust you and be open with you if things do go wrong. That also means being accepting if this goes right for her.

I do wonder whether you are misinterpreting what you are seeing too.

You are seeing the guy as in control of this and bringing his 2nd girlfriend along. But actually she brought her boyfriend and a girl to dinner at home. Is your daughter also trying to tell you she is in a relationship with the girl too? Was it her boyfriend and her girlfriend?

It does seems odd to bring the girl to meet you if she was purely his girlfriend. Maybe she is more to your daughter than that or maybe she was trying to get a rise out of you? Or maybe she just wanted you to understand how the Polyamory thing works?

I do understand your reluctance around this, and your likely worries for your daughter. It’s easy to see her as being the “victim” of her boyfriends wants. But she does have agency and she has chosen this lifestyle - so I think you have to let it play out and be cautious about expressing your concerns too hard as it may push her away.

Be there for her - it’s very important to keep being her support network and not inadvertently isolate her if you disapprove of her lifestyle. You need to be the ones who are there for her no matter what and where she comes for advice and support if this does goes wrong.