Hello and thank you for your question,
I am sorry to hear your friend has lung cancer.
Coughing is expected with someone who has lung cancer, however, if this coughing is rather new or increased in severity, your friend may have pneumonia or bronchitis and may benefit from a course of antibiotics.
Back pain can be from a several sources. Your friend might have just pulled his back causing a sprain and pain or, in the worse can scenario, it is possible that lung cancer has spread to the bones, in which case your friend will also have the pain.
If this is a new type of pain, your friend should get an x-ray of his back to see if a source of back pain can be identified. He might also need a prescription for a narcotic to control his pain.
I hope you find this information helpful. Please feel free to follow-up with any other questions you might have.
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Best wishes and good luck,