Take away the psych diagnoses and substitute a heart attack for diabetes and that was me 12 1/2 years ago. I los some weight, but didn't get serious until 4 1/2 years ago. Eat less, move more, that's it in a nutshell, but there's lots of devils in the details.
It has to be something(s) she can work into and make the basis for a new lifestyle. I don't think "cold turkey" is a good idea, because it isn't a sustainable lifestyle, nor even a stepping stone toward one. Go for portion control and balance, and cut down (or out) stupid calories like sodas and candy and such. Don't try to run a half marathon on Day 1. Do a 20 minute walk that is challenging, and gradually increase speed/distance and time. Who knows, maybe in 6-12 months she could walk a 5K (3.11 miles) in less than an hour.