Hmmm… good question. Maybe you could gently explain that while working with trainer “x” on these horses you came to really appreciate that training style and you really like how the horses are responding to it. Tell her that you believe that trying doing things contradictory to that training could confuse the horses or slow down their progress. But be sure to explain that you do indeed appreciate her and her work with horse, just that you want to continue method “y” from trainer “x”.
If she is a friend she should respect your thoughts and decisions about your horses.
Be sure to emphasize that it doesn’t mean you appreciate her help and want what is best for the horses, because she may feel jilted by the fact that you are seeking advise elsewhere.
I recently returned to horses after years away, but was very experienced back then before life took me in another direction. And I also had a friend who was similar, but she has far far
less experience as well as she is not horse minded at all! And she was outright telling me what to do and “correcting me.”
It was touchy for a bit, and I had to be tactful, but we have been able to move forward probably due to the fact that we have other things in common.