Ah, read-reading the original post, the confusion was all mine

Having witnessed it fairly recently with aged relatives, I would only wish UK national health care on a frail 91 year old on my worst enemy, and TBH that's what you'd likely get if you had something very serious like another heart attack (I believe few private hospitals are set up to provide intensive care)
Getting her back home might be interesting as well, so you'll have to take a view on how healthy she actually is to take a realistic view of any likely problems that might occur. With no insurance, if that's the way you go, repatriation may be effectively impossible. Even with it, it may not be possible.