Personally I don't get involved with someone if I don't respect there preexisting relationships. I say that because it was something I learned the hard way. Also it just feel yucky to me. I don't want to be routing for their relationship to fail. I don't want to be worried about my own motivations. I don't want to be involved. For me it's not about how much I personally like or dislike their partners it's more about respecting how they handle their emotional shit together.
Having partners whose preexisting partnership I really respect has worked out very well for me.
Also just as a heads up... you don't -really- know what is going on. He is showing you one side of the story, you are hearing the parts that bug him and annoy him (long term partnerships can have tons of that stuff and still be healthy and strong) and not hearing about the good things or her side or things.
Another personal rule I have is that I don't want my partners ever telling me “i just want to be with you” sentiment when they have other partners. I feel like it's very dividing and puts me in a strange uncomfortable place. Also talking about that can help a partner know that this isn't the kind of comfort or attention you are looking for or enjoy. Sometimes the monogamous mindset gets a little stuck on the idea that reassuring your partner that they are the best, and that you are into them more then any one else, is the kind of reassurance you -want-.