Here's a link to the Hill's prescription diet for liver problems:
http://www.hillspet.com/zSkin_2/prod...=1127858590089
Here's a link to a homemade diet:
http://www.vet.utk.edu/clinical/sacs...Liver_diet.pdf
Here's a link to someone's experience using the Hills diet:
http://www.webhometown.com/mr/liver_diet.html
The main object is to lower the protein level and have high quality fats with added lactulose. Also it is best to feed small frequent meals throughout the day so as to not overload the liver. Give our best wishes to your friend.