That's the lamest excuse I've ever heard: "The horses like the manure and it keeps them warm." It sounds to me like he's just LAZY. This is not true and your horse could end up getting thrush/hoof infections/re-infestations of worms. And she should get colic because no horse wants to drink icy water and if she doesn't drink enough, an impaction can develop. You should try to see if there's a good barn somewhere else and board her there. Where I live, $250 is a fortune to pay for boarding and when I used to board, I paid $175 a month and had the stalls mucked/the horse fed and watered/ and meds administered if necessary. You should get her out of there ASAP.
John 14:6 - Jesus said unto him, "I am the way, the truth, and the life: no man cometh unto the Father, but by Me."