If in fact you are talking about alleviating symptoms with natural remedies, then the title of this thread is misleading. There is no "cure" for MS, natural or otherwise. The drug your mother is on (I'm assuming it's either Avonex, Betaseron, Copaxone, or Rebif) was never advertised as a "cure" for MS, or even a "cure" for MS symptoms, but instead was designed to reduce the number and intensity of exacerbations. Each exacerbation can lead to significant disability, so anything one can do to slow that down will lead to a better quality of life.
Your mother could only have been on one of the drugs for the last 15 years, as they weren't available before then. Who can say how well off -- or significantly impaired -- she would be now, having had MS for 30 years -- if she hadn't gone on one of the drugs? I've had MS for 24 years now, myself, and am absolutely convinced that the Betaseron I've been injecting for the last 14 years or so has significantly slowed the progression of my disease. I have all the symptoms I've always had...but no more, and not worse. That's a GOOD thing.
If your mother does start ingesting natural remedies, even vitamins, or if she dramatically changes her diet, she needs to tell her doctor, as some of those "remedies" can have adverse effects when mixed with any prescribed medicines she might be on. And you certainly don't want to encourage her to do anything that will boost her immune system -- our immune systems are already in "overdrive", and those things can cause all sorts of problems.
You're asking about food allergies. Has she actually been tested for food allergies, or is she complaining about some foods "not agreeing" with her (more appropriately called a "food sensitivity). There is an increase in food allergies all over society today -- celiac disease (a wheat gluten allergy), allergies to peanut butter, lactose intolerance (inability to digest milk), are some of the more common ones. Any of these can make a person quite ill, and sometimes with various bowel and bladder problems that are common with MS can cause increased problems there. But she should be properly tested to be sure she knows what it is, and how to treat it, because some of the allergies are rather simple to treat, others would take a huge amount of effort to change diet, and if she's just assuming there is a "food allergy" and then goes on one of these fad diets, could worsen her situation.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....