Most store-bought soups would have too high a sodium level for dogs (and for people, for that matter

).
For fussy eaters, my vet usually recommends boiling some chicken or hamburger and mixing it with cooked rice. Sometimes meat flavored baby foods.
If you're pet-sitting, though, it might just be nerves that make her have less of an appetite. Be careful trying different foods and tablefoods, as that can upset her stomach, too. But bland foods (like the meat & rice) should be fine.