I suspect this trainer may just be childish enough to bad mouth. But the thing is, let her bad mouth. At least you can stand confident knowing you were the bigger person. I am not a fan of feeding fire with fuel. I like to let the fire burn itself out while I stand back and watch, knowing I handled it the right way.
I don't like to burn bridges either. I will let others burn the bridge, but I refuse to.
I also don't like back stabbing and sneaking around. If I am not happy I will tell the person and I will move on. Sneaking around just makes you look bad IMO. Best thing you can do is be the bigger person and the person who is up front and honest and lay it all out