Hi there and thanks for your question.
It sounds like you are having a frustrating problem with your daughter's dog!
There are a few things I can think of that may cause these symptoms.
First of all, this may just have been a very strong bacteria that needed a longer course of antibiotics. If this is the case, then she should do just fine.
Next, you may want to ask the vet about the possibility of hypothyroidism. Dogs with thyroid problems can have repeated skin issues.
Some allergies can make a dog prone to skin infections as well, but it would be quite unusual for them to come on all of a sudden like this.
Most likely this is just a nasty bacteria and will take several weeks of antibitoics in order to be cleared up. If it is not clearing up you may want to have a culture done of the infected area to make sure that it doesn't need a combination of antibitoics in order to heal.
The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would highly advise contacting your regular veterinarian.
A bacterial infection certainly could cause these problems. If the dog scratched at the infected area and then scratched at her ears she can spread the bacteria....although I must admit it is not very common.
The lethargy and inappetance would go hand in hand with a nasty infection and should clear up within 1-2 days of treatment.
However, there is a possibility that there is something underlying which is affecting the dog's immune system. It certainly wouldn't be the normal presentation of lymphoma and cancer really wasn't a consideration for me when I answered the question.
If this were my case, I would see how the dog responded to antibitoics and if she is still not feeling well in 24-48 hours I would do some bloodwork to see if I could detect anything else going on.