You can't fix those horses, they aren't yours. The people who own them have the responsibility to fix them. Its not hard to destinguish a skinny horse from a healthy horse. Your BO new this was happening, so don't let her guilt you into "its your fault".
I left a barn a couple months ago because 4 mares were old as dirt and weren't being cared for properly. As the trainer and co-manager, I stepped in and got these horses a farrier trim (they hadn't had one in 3 years!) and had my @ss reamed by the BO. "Why did you spend that money?! They are useless and don't do anything!" I told the BO that I scheduled the vet because they looked terrible (extremely skinny, long, scruffy fur, ect) and told her that if the vet couldn't do much, I'd like her permission to put them down. She was furious and had me cancel the appointment.
So I left. In caring for an animal that is that ill, malnourished and suffering, in essence, you are allowing it to happen. I couldn't have my named attached to that farm if animal control was called (you can see these horses from the road) and I wasn't allowed to do anything more than what I had already done, so I left.
If you WORK there, simply say "Thanks for the opportunity, but its time for me to move on". You don't have to go into an explination. If you simply ride there, just don't come back. If she asks, reply "I found another barn. Thanks though." If you board there, give her your 30 days notice. No reasoning is required.
She is already pointing fingers and spreading the blame because now the heat is on her. Get out before it gets messy and she drags you down with her. She will either pin you for these horse's care, or she will blame you for what people are saying about her. Either way, it will not end well at all.
Just leave, and be quick about it.