I was taking Lisinopril for probably 6 months and I started coughing. I would cough when I was asleep. I went to an ENT doctor to see what might be wrong with my throat. He did not find anything wrong except it was irritated and told me to gargle with vinegar water. The next time I went to my Internist, who had prescribed the Lisinopril, I told him about the cough and how much it was disturbing my sleep. He said "It is the Lisinopril". He had me stop the med and the cough went away.
Look the drug up that she is taking on the web to see if cough is listed as a common side effect. Before you do this though, don't get overly concerned with all of the side effects you see related to a drug when you do search. All possible side effects observed when trials are done on drugs are listed even if they are not caused by the drug. Some blood pressure drugs do cause cough though in many people. The drug with the most likely side effect of cough as I understand it is lisinopril.
Good luck,
Rob
Tell your mother to tell her doctor and he can switch her to another BP med.