Hi!
i can’t see that you have attached any pictures. If you’re having trouble attaching them, you can upload to an external source like imgur, picasa, flickr etc and paste the link here.
First question is: Are ther any round firm poops at all or is all of it runny and mucosy?
This is important because this is how you differentiate true diarrhea from sticky/mushy poop. True diarrhea is a veterinary emergency, mushy poop allows you more time. If all of it is runny and mucosy and there are none of the round poops, it’s a veterinary emergency. He can get dehydrated really quick, so he’ll need sub Q fluids and an appropriate antimicrobial.
There is a rabbit-safe dewormer called Panacur (fenbendazole) that kills most worms that affect rabbits. It’s OTC.
Since I can’t see the pictures and his general demeanour isn’t that of a happy healthy bunny, plus you have to leave him in your friend’s care, I can only say it’s best to call a vet and book him in even if there are some round poops. (If all poop is runny and mushy- call a vet asap.)
Your friend can come here and ask about further care for him while you are away. There are more or less always people here, and we love helping out bunnies in need.