Your fish will need water changes and some limited feeding over such a long period of time. Fish can go for very long periods of time without eating, but even if they do not eat, they will still produce ammonia.
It would be a good idea to write up detailed instructions for your friend--be sure to discuss that the new water must be conditioned and the same temperature as the old, that the fish must be slowly acclimated to the new water. When I asked my mother to take care of my fish, I had her practice feeding and changing the water while I was there to help/oversee her, just to make sure she did it right, this gave both her and me some confidence.
Also, people who don't usually take care of fish tend to go a little nuts food-wise. I would pre-package the correct amount of food in little plastic baggies so that you are sure your friend doesn't overfeed. Feeding less often than you would normally may be prudent, as it will reduce the waste your fish produces.