ANSWER:

If the illness is chronic and there is no possibility of recovering from it, a person should feed one poor person for every day missed. If it is not chronic and they predict she will be ok in few years, she must make them up later and there is no fidya. 

My mother has to take many medicines as she is suffering from depression hyper acidity and few more things doc has advised her to not to fast this ramzan so whom can she the fidiya