Since you asked, I believe that he wanted her (more to the point her horses and her money) to stay, but on different financial terms more favorable to himself.
If he could have rid himself of her, but retained access to the horses and money, then sure. But he was stuck in a love/hate relationship (love the $$$$/hate the client) and he couldn’t legally get her to leave while still getting her money. I am baffled along with everyone else as to how his attempt to murder her was going to solve the problem.
But no, I don’t think he ever asked her to leave. He could have filed an eviction action against them, but there is no court record of his having done so. He could have changed his training services so that during training rides, he sat on her horse at a walk while coaching his girlfriend, or by sitting by mutely during lessons while drinking Long Island ice tea. I have had several trainers get rid of me by simply providing crappy training services, and I don’t think they were trying to get rid of me.
Trainers “fire” unwanted clients all the time. If he tried to fire her (ask her to leave the training program) and she refused, I really cannot imagine a top trainer having trouble getting rid of an undesired client by simply providing crappy training services.