Has your mom already reached her retirement age or is there still some time left? If her retirement is at least 7–10 years away then I would advise investing the money into an equity fund. ELSS funds are best for this purpose. They provide the double benefit of equity returns as well as tax savings under Sec 80C. Equity funds usually give the best returns when invested for 7+ years. The money would grow substantially in a period of 10 years.
On the other hand, if your mom has already reached her retirement age then do not invest the money in a market-linked instrument like equity. It’s better to go with safer investments at this point. I would suggest investing the money into an SWP(Systematic Withdrawal Plan) of a good debt fund. That way every month she would be able to withdraw a certain amount for her expenses from the fund. I hope I was able to answer your query.