Hi is your friend taking statins, some people develop a try cough with that kind of medication. Also is your friend overweight? If so he could be suffering from acid reflux which can cause an irritating cough too, just mentioning this because one of the members in our group used to suffer from this, he lost weight and is fine now, but it took ages and chest xrays and throat swabs and all things like this to evenually find the cause.
Look for Potters herbal cough mixture and pastilles (Potters are famous for their herbal remedies, they do sugar free verions)
Check this link too
http://www.haelan.co.uk/Remedies-ColdSeason.shtml and Romola is absolutely right, Thyme is a traditional cough remedy, just make it like tea, sweeten if it you want to and drink that 4 times a day, maybe back it up with some herbal pastilles for in between to keep your throat lubricated.
You can get herbs from
http://www.spiceworld.uk.com/
All the best
Karen