She needs to get in to a vet, and absolutely, basic testing needs to include a basic blood and urine test.
A very skinny boxer, at this age, with appetite issues could be caused by many problems including:
- intestinal parasites (especially the microscopic bug, giardia)
- pancreas problems (exocrine pancreatic insufficiency)
- inflammatory bowel disease
- a chronic gastritis or intestinal problem
Very sorry she is not feeling well. No chance to use over the counter therapies in this situation. I think every delayed day is going to be costly, because she will not only lose weight but also muscle mass.
The basic testing will help the vet to start quickly ruling out some possible explanations for her illness.
Well, it could be related to mouth problems, complications in the stomach, or organ disease (kidney failure can cause a very bad odor).
Wish I could do more. Definitely, well worth basic testing.
Hope that information helps