I'm almost positive this is another way SJM is not-so-subtly referencing Cassian and Nesta being mates. Nesta doesn't want anything to do with Cassian, just as Feyre didn't in acotar + the first third of acomaf. When Rhys says he'd "seen it happen," I think he means he'd seen it happen to him, when Feyre had made it abundantly clear with her shoe-throwing etc. that she didn't want anything to do with him.

I think, though the writing is a little confusing, the "had felt that way plenty" part is actually saying Rhys had felt the way Cassian is feeling, plenty. As in, dejected, longing, pining, but keeping a respectful distance. Because, again, he felt that for Feyre. That's how I've always read it and that's what makes sense to me, but if you have other thoughts I'd love to hear them!

Well what I was thinking is that after something so traumatic Nesta couldn´t see Cassian in the eyes bc it reminded her of the moment when she coverd his body with hers and made it clear that she loves him so much that she is willing to die with and for him + just to remember these horors was reason enough. Rhys can actually relate to both sides. He saw Feyre die and he probably couldn´t look at her without remembering the scene under the mountain like most of the time and on the other side he knows how it is to be decived from the love of his life.