There are several things that come to mind -
1 - FOOD - many good quality foods are in the recall and causing symptoms like what you have mentioned. You can do a basic cooked diet for a while and see if the symptoms go away. 1 lb cheap hamburger and brown rice and stewed or whole tomatoes (the stuff around the seeds in the tomatoe are pectin and are binding) cook it until it is mush and you can't tell that the rice is rice. Gradually introduce regular food.
2 - For a dog that has been suffereing from digestive issues you want to replace electrolytes. Pedialyte is a good option. add a little to water or to whatever food you can get the dog to eat.
As far as diagnostically - Pat gave you very good advice - good solid blood work. Full thyroid panel is a must. Food allergy tests could be in order but switching to the diet above and adding things a little of a time could also at least rule out some things. Skin scrapings for Mites would be in order AND have you considered a Flea Allergy. Having lived in a state with a lot of fleas and now in MN where at least it is cold enough so they fleas sort of die off in the winter - less of a problem - I forget how irritating they can be unless I'm traveling out east or south.
There is one other thing that I've noticed a lot lately with some animals that are from less than stellar breeders - a large increase in what is being dianosed as Irritable Bowel which looks a little bit like Malabsorbtion to me.
Hope that helps a little - tell her good luck.