I am so glad she has someone to love her. I would start at the begining, just as if she were a young foal, basics,basics,basics, halter, ground work , slow and gentle. You must get her trust first before you can do anything, Lots of grooming it is a great way to bond and get her use to your touch.
Start with small amounts of grain and work your way up , not too fast it can make her sick and add to a lot of other problems you don.t want. ALWAYS fresh clean water that she can get to at any time, helps to wash the toxisen out of her body and hydrate her.
Pasture and grass if that is possible , but good horse hay is great too.Oh and all the love you can give.
Check with your vet about haven her teeth floated, HELPS a great deal so she can get the most out of her food, and wormer, every 2 months at first, then I worm every 3months years round.
From the looks of the pic you have just saved her life. God bless ya .
If God made anything more wonderful than a horse .. he kept it for himself.