Charlie, I am so sorry for what you and your wife are going through. I know it must be miserable for her and frustrating for you, not being able to help much. I do know that people with diabetes have a very difficult time healing. Most amputations are caused from infections that wouldn't heal. Add to that fact that the mouth, being moist, has difficulty healing. I do have one suggestion: The shakes are a good idea, as are smoothies with peanut butter for protein and fruit. Serve them with straws. If she can drink through a straw, it will be less hurtful to her mouth. After she eats or drinks anything but water, have her rinse with either plain or salted water.
Other than that, just do the best you can and come here when you are able.
Charlie, I have been out of the loop for awhile so I was catching up on things. Many years ago I had to have my jaw wired and they way it was done I got alot of cuts on my tongue and cheeks. They had me swish with hydragin(spelling issues so I dont have my glasses on) peroxide with small amount of salt and warm water it helped but was also soothing