Rules lawyerism at worst. Tell your friend that he should be ashamed of himself to even think something like that. Tell him to use some common sense. He's wrong.
However I can't find the specific rule either, although I'm 100% sure you can't use companions in a battle in any way if they are still in the fellowship.
Best i could find right now was in the FAQ "the Fellowship, and any Companions still in the Fellowship, cannot be destroyed in combat."
If this were a certain Finnish icehockey forum, people would suggest you give your friend some skirt fabric .