If someone has lung cancer, would standard over-the-counter chesty cough medicine ease their coughing?

It may. The cause of the cough also might not be the lung cancer. My hope is that the patient can find relief. If the diagnosis is uncertain, don't make any assumptions. A treatable cause, especially if the person is a smoker, might be identifiable. Even if this is not a treatable lung cancer, palliative care may find a way to help.
Call your doc. Call your cancer doc, he will have the best suggestions as he knows what is going on exactly as well as what other meds you are taking. We don't know enough about you and the disease, based on your question, to be able to give sound advice here.
Possibly, but your symptoms will probably best be managed in conjunction with your physician.

Related Questions

My mom is a smoker. She has had a non productive cough for about a year now. Today was complaining about upper back pain. Can it be her lung? Cancer?

May need some Tests. Persistent cough can be due to smoking(bronchitis) or other lung diseases. Since it has persisted for one year, you should get a medical check up to find the cause. A simple chest X-ray may reveal any obvious problems but a CT scan of the chest is even more detailed and more reliable test to find a cause like Cancer(which is unlikely) in the lung or in the back bones. Read more...