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.
Well I don´t know for certain but he may be showing people the characters had, have or will have week spots for. Or just a person who doesn´t look as terrifying as his true form. Idk but I am certain that we will find out more about that in the next book (a court of silver flames).