It sounds like multiple traumas and difficulties in your family are finding a symbol in food and eating and sitting at the table.
You say "my daughter's ex used to force her to eat," do you mean the ex forced your daughter to eat, or the child? If the child, no wonder it takes her a long time to get a meal down.
What is going to happen when the ex gets out of jail? How did he get such a light sentence for abusing his girlfriend every day for a year and a half in front of a child? Does your daughter intend to stay in touch with him?
It seems like your daughter needs to set up a better life plan well before this guy gets out. For that, she should probably see a counselor. If she gets some forward-looking plans and begins to act on them, she will probably stop using food and dieting as a surrogate for the other controls she has lacked in her life. Anorexics do this -- everything else is out of control, but their eating is in their control. So they stop eating. You don't want your daughter to screw up her eating in an attempt to feel in control of things that don't have anything to do with eating.
In the meantime, try to take the pressure off mealtimes. Put out healthy food for your granddaughter and sit with her, but don't watch her and don't bug her and don't coax. (Lots of kids eat more lightly than their grandparent thinks they should. My son, who had no traumas in his life, used to eat so little that my friend (age 65) would mutter, "Not enough to keep a gnat alive." It should not be your job to judge the child, just to place a meal before her. After a given period of time, put the food away. See that the child is getting a lot of exercise, and pretty soon she will develop an appetite if she doesn't associate food with anything more than satisfying hunger.
Good luck, it sounds like a mess, and you're going to have it in your face for a while. You could benefit from some counseling too..
I agree with AnnieBrooke and would also like to suggest a CASA (Court Appointed Special Advocate) volunteer for the family. They deal with the children in domestic court cases - and I would guess this man will be in family court for visitation rights - or to have rights severed - when he is released.
Go to CASA.org, and look up your local court system, and give them a call.