i'd think if a dog with untreated mitral valve disease had kennel cough, it would likely put additional strain on the heart, to be coughing that hard.
It's good that you are helping your friend find out more about what might be going on, it may result in sam getting the help he needs sooner.
Charleen and Karlin and Rod, thanks for those informative links. Charleen, that link you posted is the website of my vet, or one of my vets, the one that i started taking my cat to years ago, and the first one i took zack to when he was sick when i first got him. They weren't the vet that finally cured him, but i still have confidence in them for many things. I've found lots of detailed information on their site, their info on medications is a good resource. Their info is so clearly written and thorough. I learned a lot about kennel cough from reading it just now, and also from the Mike Richards Q&A, he's also a great communicator, so clear and straightforward, and caring. i haven't yet read the MVD links--so much to learn!
i hope sam will be ok and if he has kennel cough, it will soon clear up on its own. it sounds like the sooner he has a competent cardiology evaluation, the better.