I’m so sorry for your sudden loss of your pet, and the added stress of not having been told earlier.
Although one has said one should be contacted we are just people. Also in other circumstances it is usual that in a situation of crisis we don’t know for sure how we will react and if we will be able to follow the protocol we have previously been told or learnt. This is true also in professional environments where people have the education and experience specifically to deal with certain situations. It is not enough. That is why so much time is spent on practice, emergency drills, re-certifying certain skills etc., and why one in some professions even is able to loose jobs and certificates if it is not done or one doesn’t pass.
This doesn’t help you one bit in general. But if one has this as a background, it can open for an understanding and a conversation that will help assess the situation and find the solutions that is needed. The emotional response to a situation - for instance “why wheren’t you careful parking in the garage! I’ve told you so many times to mind your angles!” will not solve the problem (car damaged), give an understanding of what happened (I was busy looking in the mirror to mind my angles, but the pace was a little to high and I didn’t check the other mirror fast enough). and it won’t open up for a conversation on how this can be avoided in the future (drive slower, look in that mirror by that pole). I’ve crashed my car myself, and even that which is so minor in the big picture - you might have learnt what to do, but when you stand there with the result, other driver, other people watching… you can’t prepare fully.
I can’t imagine how it is to get such a hard message while on holiday - or find a pet deceased as a sitter. It is true that rabbits and small pets can die “suddenly”. But as I’m sure you know (mention it for readers that might experience similar), small pets and rabbits can also go a long time with health issues, because their instincts are that they are prey and they must hide any sign of weakness. So the rabbit could even have been ill for quite a while.
But I’ve had to take life and death-decisions for my own pets, also where it was not clear cut and I had to choose between lenghty/costly procedures and letting my pet go. But I can imagine that I would have taken it a lot - a LOT - worse if it was looking after someone elses beloved pet than my own beloved pet. So there is a possibility that your sitter also is in quite a state, hasn’t had any drill, and handled an emergency badly. I do however have an outlook on life that everything has a reason, and people in general want to do good. If the result is not good, then it is nevertheless true although they fell short.
Lots of words in short; I think it will serve your interests to maybe hear a little about her thought-process. It could be fruitful to have an open entrance to the topic, saying that one realise this must have been quite a shock for her also and a difficult situation as a sitter. And maybe let HER suggest what the two of you can do moving forward and how she will be best assisted to be able to sit for the rest of the period (or if she would like to cancel). I think it will serve your interest best because I think it will be the same result, but you will by your actions of reaching out lay the foundation for her to not be afraid, and that in turn will give you more peace of mind. That is my experience also from my work that we often get better results if we let the other party suggest solution (-s). They will feel more empowered and will be better able to execute.