I don't think it's a waste of time Archus. She doesn't believe her diet is a factor and if the test can prove she's wrong it will make her rethink herself. Awareness is a necessary step on the way to recovery if we want to make it permanent and not be susceptable to more crap in the future because we disregard it as an influence today.
In other words she could easily find herself feeling much better after coming off the meds, and feeling better may take that as evidence that cake is good and has no ill effects at all! Better for her to find out now it's effects than 20 years down the track and many more problems in between.