What else, besides ride him, does she do with him?
Once in awhile, I'll just give my horses a good grooming and some hand grazing time. It breaks up the, 'Oh here she comes, it's time to work again' monotony.
My Arabian gelding HATES ring work. After about 20 minutes he's pretty much done too, and he's only 12 y/o.
She can even walk him around the acreage on a lead line. Kind of a walking trail ride.
Mix things up and keep them interesting for him. He sounds like a kind hearted guy, and all he's telling your daughter is that he doesn't want to just do patterns in a ring.