Please suggest some medicine to treat cough(side effect of letrozole)?

I am sorry about what your mother is experiencing, I suggest you give her Dr. a call and ask him / her what she can take safely (the Dr. knows her history, like allergies, etc.), without interacting negatively with the medications she is on, and I would call the Dr. right now if I were you.

I hope your mother gets well soon.

Take care,

maso.-

She ought to go in for a visit. It is possible that she may have a pneumonia considering her impaired health. Increasing cough is one symptom that ought not to be ignored with cancer treatments.