Will an OTC cough remedy help Mom's cough?

My Mom has been diagnosed with lung cancer; a walnut sized tumor in one lung and an avocado sized mass in the other. She has had x-rays and a CAT scan but no biopsy and may choose not too. It has metastasized, according to the person who read the CAT scan, but has not spread to her adrenal glands, yet. However, her lymph nodes are involved. I don't know about the brain without a PET scan, apparently. She is coughing a lot. Her nurse practitioner recommended she see an oncologist for palliative care, but I don't know if she will choose too. My question is, will an OTC cough medicine do any good for her? The nurse practitioner seems to think it will. If so, what kind should I get her? The other question is, will an oncologist prescribe a cough medicine for her without actually meeting with her or making her have a biopsy, which she will probably refuse to do anyway? Is there any way to tell what stage she is at without a biopsy of some sort?