Personally I think you need to check out your 'status' with him; would he value/defend/sustain your relationship if his wife does open sexually with him again. I'm not suggesting he wouldn't but I think you are wise to be unsure, and quite possibly he/you won't know until it actually happens. I suppose really I'd just advise you to be aware of the possibility and that you could find yourself frozen out. A lot depends on the strength of both your emotional tie with him and their commitment as a couple to polyamorous principles (as opposed to expediency)
"Several years later... she flinches if he tries to touch her." There are obviously powerful unresolved issues, pain and frustration going on between them as well as a powerful connection.
On the theoretical level I do think it's unrealistic to see sexual love as beyond jealousy or competition. That smacks of 60's luv'n'peace naievity. All of us, mono poly or whatever, have the contradictions of both love and hate in our make up. We want: to belong and be separate; to possess and be free; to be secure and dangerous. It will serve us to recognize and accept these contradictions, in ourselves and our loved ones. Sexuality can be wild and ruthless. That's what makes it exciting! And what makes it fraught with emotional dangers!
Talk to him, talk to his wife, talk to your friends. Recognize that any and all of you could be hurt or could hurt each other and, ideally, create a respectful friendship that will transcend it. And enjoy it all!!! X