So they may or may not be “a nice person,” but they are the sort of ignorant rash uncoachable client that is fired by their trainers. And they are manipulative and they lie. These kind of clients will destroy your reputation. Walk away now and be more careful in future.
Once you fire them as clients all your mutual acquaintances will feel free to tell you how they really feel about them.
The longer you stay with them the more you look like you are a trainer who is also rash dishonest and manipulative. It doesn’t matter how you cut if off so much as that you do and you aren’t suckered into helping sell a lame horse to an innocent their party.