There are a number of things given to patients here in USA to treat the mouth sores -- the most effective seems to be Gelclair (which is a pain-relieving coating agent) and then there are a number of mixes which contain pain-relieving and anti-inflammatory (and often, anti-microbial) materials such as Mile's mixture, "magic mouthwash" etc.

As you are in India, ask about the availability of benzydamine hydrocloride (Difflam) which is available over-the-counter in UK and Australia, and by prescription in Canada and many European and South American countries. It is a NSAID which has been found in clinical trials to be the best to relieve pain and shorten duration of mucositis. It is not yet available in USA.

Finally, pain relief will probably be necessary and this is usually some fairly powerful opiate so you MUST be proactive to get a pain-management plan in place so your mum can continue to be able to swallow and stay hydrated. In USA a lot of people get a stomach (gastric) tube to enable feeding and water while their mouths are really sore. But they still need pain relief for basic quality of life.

Hope this helps, good luck,
Gail