There are few dental diseases that could cause a sistemic rash like the one you describe, but they are specific diseases that give many more symptoms that are very characteristic and are easily diagnosed. So i highly doubt that your moms rash has something to do with the dental procedure.
You do not provide basic information about the history of the rash. Dermatological diseases require a close up, physical clinical exploration. I suggest to seek a dermatologist consult. Many dermatological diseases look alike to an unexperienced eye. Skin lesions are common causes of headeache to med students. Dentist are over specialized on thier area so they won´t help you with her disease.
So look for a trustworthy dermatologist. Do not waste more time.