Hey Elmek, how is your friend's dog doing? Hope he's OK. Just to say that Jack, our Collie, is gluten-intolerant, and after a while of illness it was finally whittled down to that diagnosis. We started him on Burgess Supa Sensitive complete dry food. It's about �20 for a 15kg bag, also available at Pets at Home (not sure about other stockists). He likes it, and it lasts. A Rottie will of course need more than a Collie! But a bag does last well. They do sml bags too (2.5kg or similar) if your friend just wanted to test it out for him.
I found it really hard to pick up gluten-free treats for Jack, as there was about 3 available in the entire area!
We found a great website, Oscars, they do food too but it's a bit expensive. They have meaty treats and biscuits, which Jack loves, and you can buy in bulk as they last a year. The bigger sizes come in those takeaway-style plastic cartons, so stay fresh. They do charge for delivery if your order is less than �25, but if you have a local Oscars agent they tend not to charge you delivery, as long s you go through them.
Bit of a wordy answer I know, I just remembered how difficult it was for us at first, faced with a food bill that is more thn we pay for our own groceries!! As long as you shop around, and find something he likes, he will be back to his normal strength and size in no time. Keep us posted! Lisax