Your grandmother is 82yrs old, does she have any health complications moreover what type of infection is she revering from?
In general you can give her clear veg soups, mashed rice with dhal( dal can be seasoned) , soft custards, carrot juice, tomato juice( salt and
sugar addition will depend on other health complications which she may have.
Next Steps
A health supplement especially for the elderly who are unable to eat I can suggest Geritovita( Holin brand ) the products are millet based infused with ayurvedic botanicals).
For further queries get in touch through Practo