My fwb’s gf is mono and even though he’s been with me and his wife since before the gf got involved, she’s always complaining about how he must not really love her because he won’t leave me and his wife for her. I’m trying to be patient, but damn :(
That sounds like a tough situation - but not one you can do much about, or are really responsible for, unless she’s making these complaints directly to you? In that case, you have the right to be clear with her and/or your FWB that you don’t think that’s healthy or appropriate for her to say around you, and that you need partners of partners to be understanding and accepting of the terms of your relationship.
But if you’re hearing this secondhand from your FWB, there’s less that you can do. If you’re worried that he is eventually going to give in to her demands and leave you, bring that concern up with him, and see what he says. If you’re worried that her pushing for this is hurting him, you can speak up - it is our business when our partners are in unhealthy situations. And if you think he’s somehow leading her on or taking advantage of her, say something. But beyond that, it sounds like this isn’t your problem.
Not to say it isn’t a problem - this woman seems to be in denial and is clearly in a situation that is unhealthy for her. But if you believe that your FWB is doing things in an above-board way and this isn’t affecting his health or yours, sometimes all you can do is step back and let other people make bad choices.