All you can do at this point is comfort your friend. Criticisms will only hurt. Once this has passed, maybe (if you are 'up' for it) volunteer to check on her cat if she intends to be away. It can be easy to miss things (stress this to her so she doesn't become defensive) but a 'cat person' is more apt to notice.......
Treatable? Her vet will tell her that. I suspect (and hope) it will be.
Sad that, like your friend, we have to learn our lessons through the pain of innocents.
I shall be thinking of him and hoping for the best.