If she is not on any medication, look into an anti-anxiety medication for her with your doctor. This may stop the chattering, and make her feel better generally as well. You can also purchase a rubber mouth-guard (like football players use) for her to put in her mouth when the chattering is unbearable. Blessings - Blu
Has your mom talked about her anxiety to her Dr yet? If not, I would recommend her doing so. Her Dr may be able to recommend a therapist. or psychiatrist.
I'm so sorry your mom is going through this. The good news is there is help available. It could be therapy alone or therapy and medication.