Seconded. From the "poly perspective" he cheated and the two of them betrayed your trust at an extremely vulnerable time, just like from the mono perspective.
That said, what the poly perspective does offer is the knowledge that him hooking up with her does not mean that he doesn't love you any more or that things are necessarily over now. But it's still up to you whether or not you're willing to forgive him and trust him again. I imagine his chances would be better if he showed sincere remorse rather than accusing you of being too harsh...
I wonder why this happened during your hospitalization? Maybe they were both so scared for you and in such need of support that this is the (inappropriate and uncool but perhaps understandable and forgivable?) way they found to be there for each other. If you haven't already, I would ask him what the heck was up with the timing.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.