Sorry this tricky situation is occurring OP.
I'm a dog walker and also do some sitting (but not much as I have a family). Where I live (IUK) sitting is where a person cares for your dog in your own home rather than in the sitter's own home.
The set up that you describe is called boarding where I am and you must have a license to offer this service. This means your home and procedures are regulated.
One of the terms of the license is that the premises must have a separate suitable room (or secure area) available for every single dog (including any resident dogs) if required.
Often these separate rooms aren't all used unless a problem arises. But they must be available and in this situation they ought to be using them.
Boarders' official guidance also requires them to notify you (or a nominated alternative) if they feel that your dog is significantly unsettled, or unwell. Most five star facilities will insist on an emergency contact should things get really difficult. I appreciate that this isn't practical for some people, but that's what is usually asked for.
I do think that the responsible and caring thing is for the sitter / boarder to let you know that your dog is struggling, and ask if you can help with a contingency plan. A less responsible person might say nothing, and pretend all is well.
To those saying the sitter has 'bitten off more than she can chew' I can appreciate your POV but I also think that even after trial visits dogs can act differently dependjng on things like the other dogs, a build up of stress etc.
Even the most experienced dog handlers and trainers would suggest a stressed dog showing signs of reactivity is removed from the stressful situation (unless they are fear or punishment based trainers). This really is best practice if the usual sensible measures have failed (e.g. as much separation as possible, time to decompress, calming activities like sniffy walks and licky mats, plenty of rest and down time etc..). There aren't any magic wands that can stop a dog being stressed or reactive quickly- it takes time and patience.
OP you've done nothing wrong, and I think it's possible that the sitter hasn't either. These things happen.
Thoughts for the future:
A boarder who only has one dog at a time might be less stressful for your dog.
A sitter in your own home may be less stressful- familiar surroundings, usual routine, no other dogs should all help.
A kennel can work well for really reactive dogs but can also be stressful. Much depends on the dog and the kennels set up.
Whatever you do try to do several trial runs for shorter periods. Ask the person how they try to keep stress to a minimum, what they do if a dog becomes reactive etc..
As a dog walker I get to know my dogs very well. I've walked some of them every day for years and we have a great relationship. This means that they tend to be very happy when I sit for them- this might be another option.
Finally, when you get home do bear in mind that your dog may be feeling stressed. Don't panic, but do be very careful and do not push him out of his comfort zone at all. Let him decompress over the next week, keep things calm and relaxed. Allow him space and access to his safe place. Keep walks calm and easy. Until he has decompressed he is much more likely to be reactive or act in an unpredictable way. Look up the bucket theory if you'd like to find out more about how a build up of stress can push any dog into overwhelm, and how long it can take for them to become calm.
Let us know how it goes.