When I bought her the lady that trimmed her trimmed the outside of her feet and she was walking on her soles. She could not walk because of this. I only trimmed it enough that she was walking on the hoof itself instead of the soles. She was much more comfortable when I did this. She had also not trimmed them correctly and had her sitting strait up and down
Please learn all you can about hoof function. There is a lot of recent research & findings that have changed the way many think about horse's feet.
If she was more comfortable after the trim, then you likely didn't do anything too bad, but her comfort was probably more about taking her heels down to a better angle than the soles. The walls of the horse's feet are not meant to support the whole horse at all. The soles are on the bottom of the foot for a reason - to support the horse along with the walls & prevent the horse effectively being hung from her laminae, as she will be if peripherally loaded.
In addition to the above being a problem for a healthy horse, this is a foundered one, which means her laminae are already disconnected & in no fit state to support anything. Her pedal bones are likely pointing into the ground at the toe and her soles are probably already over thin, especially around the edges. Peripheral loading will likely cause further descent of the coffin bone. So thinning sole is not at all advisable in this case. Instead, ensuring the walls are relieved of pressure, to allow them to be able to grow down strongly, and padding her soles to provide more protection while they're thin & the primary support is the way I'd handle it.
He is also very good with founder in horses. He has alot of people that he has helped them get their horses back to normal or darn close. I fully trust him. And he works with my vet so if my vet trusts him how can I not!
Great if he & your vet are indeed as knowledgeable & deserving of your trust as you believe, but how do you know your vet knows what they're on about, without learning the principles yourself?? As mentioned, there is a lot of recent information come to light to change people's approaches & it's not a given that your vet or farrier is up with it all. Not saying they're not either, but speaking from personal(& clients, & heaps of people I know) experiences, there are many out there who are way out, so it pays to do your own research & not trust anyone blindly, no matter what kind of 'expert' they claim to be. Best wishes!