My wife and I are poly, but it seems like the girls she meets just want to flirt with her because their partners aren't giving them enough attention. The minute she makes them feel sexy enough they drop her like yesterday's trash. I'm so angry for her because this is just crushing her self-esteem. I wish people didn't see poly as just flirty fun and ignore the real person with real feelings involved.
I'm so sorry that you and your wife are having this experience. It is true that people in the dating scene sometimes have misunderstandings of polyamory that lead to real hurt for people trying to date polyamorously.
It might be time for your wife to consider where, and how, she's meeting these women. If she keeps getting hurt, it's okay to withdraw, spend time with people who are healthy and safe for her, and take steps to protect herself from this ongoing pain.
She might have better luck with an online dating profile that is very explicit about her expectations and needs as a married polyamorous person, or checking out polyamorous meetups, websites, and other communities. If it's something she's interested in, the kink/bdsm scene tends to be very poly-adjacent as well.