I agree that you should ask your friend to have his dog checked by the vet. But it might not be a serious problem.
Some dogs cough when stressed. I rescued my dog Xochitl from a shelter. She'd been abused and was VERY shy. My vet had given her a clean bill of health but she coughed alot when we first got her whenever she was particularly exited or anxious..which was alot since she had some separation anxiety problems for the first couple of months.
We worked on the separation problem and she doesn't cough anymore.
So, It could be, if the vet says there is no health problem, that the owner needs to spend more time with the dogs when he's home. Good for you for keeping them company!
:-) Bettina