Take control and active participation in your horse’s daily care, especially if you cannot trust the BO.
I am moving my horse on the 15th for similar issues. Too much mud. BO does not take the wetness / cleanliness seriously. I go out to the barn every day, and check legs etc to catch stuff early. Last week it was a heel abscess, which was the last straw.
I simply emailed the BO and told her that it isn’t working out for us, and that the mud was preventing me from enjoying my horse.
Take over your horse's care, make arrangements to move. In the meantime, try to get him turned out alone in the least muddy area you can.
Why does the BO and your friend need to be so involved in this? I don't understand why the 3 way with you, the vet, and BO needed to happen. The Bo clearly always thinks she's right. You'd have been better off just taking down the vet's orders and doing it yourself rather than trying to convince the BO. It's clear the BO doesn't give a crap.
The friend is involved because I asked her about some concerns and whether I should call vet, etc. That is how she knows the ordeal. As far as BO, I agree. We had vet give BO her orders since she wanted horse in certain area that was dry. The biggest part I forgot to mention is that there are 9 horses in his field. It's probably 4ish acres. Another horse comes Sunday. She only has 2 fields, both overfilled. With him being on stall rest now, I can do the meds myself and treatments. Just tough with my full time job. I knew if I told the BO (who also manages place) what the vet said she would not comply with vet orders so I figured best coming from vet than me (the over reacting horse owner). I never expected her to be so disrespectful to the vet. Definitely going to move. Actively looking up places now. I have a friend willing to take him until I find a place (she is 36 miles away) but she is willing to do his morning treatments and I'll handle evening. I guess the catch is coming up with the excuse as to why I need to move him asap. I'm a horrible liar.
You don't need to "come up with the excuse," you need to be honest. What are you worried about, hurting her feelings? So long as you're not breaching a contract and you were fair with this month's payment, you should just show up with a trailer, load your baby up and get the hell out of dodge.
If you cannot leave soon step in to do as much medical care as you can, tell the BO you'd appreciate her help because YOU are very, maybe overly, concerned about your horse's health and keep looking for a new barn. It is to you and your horse's benefit to play to her weakness (she knows everything) until you can gracefully move on. There's no value in alienating her now or when you leave and maybe, if she thinks she's being the hero she might help you find a drier barn. But you are right about her not providing the care and support you and your horse need and deserve.
There shouldn't be any reason to lie. Tell the BO honestly that the turnout situation is not working for this horse and that you don't feel that this is the best situation for him. Personally, you should have been concerned when she wasn't concerned about his health. And no horse should be standing in consistent mud that deep without some sort of daily care to clean up the legs and help keep scratches away. I'd be more surprised if he didn't have scratches.