I agree with SR. Why bother with her drama? They are your horses, not hers. If she's pissed, well, that's a waste of her energy. If it's too much of a problem for her to be mad with you, you could always tell her (as politely as possible, though it may seem hard) that "I wanted to give another person an opportunity, I'm sorry that it's not okay with you. However, I feel that since Sterling has worked with Cash before, she is more capable at the moment. I am not against you working with my horses, but, right now, you are not the right person." You could throw in that she can talk to you about working with Cash, if you're comfortable with that option, that is. Good luck!