I think this is flea larvae. Had them in my old house when I lost control of the flea situation (we had neighbourhood cats coming in) - found them on my bedspread where the cats were sleeping.
Indorex spray will sort it out - kills all life stages of the fleas for 12 months in the suroundings while you work on the cat. Get it on-line from Vet UK.
http://www.capcvet.org/images/Fleas/fleas2.htmAdd. Just re-read the original post about the cat being jumpy and not wanting to be on the floor?
Definitely fleas in the house. Poor mite is panicking as he clearly can see what your friend can't. I'd be surprised if grandson hasn't picked up a bite or two if he was on the floor, but it won't hurt him.
Spray everywhere then hoover, chuck out the bag (outside in the bin) then spray and hoover again. Concentrate on the warm areas like under radiators, bedding and soft furnishings, and make sure the cat is treated with a good topical flea treatment like Frontline or Stronghold to prevent re-infestation.
Need to treat the cat for tapeworm, too. The eggs of tapeworm are transmitted by fleas and any sign of fleas guarantees worm infestation too.
Buy all these things on-line from Vet UK. If you use the link from Purrs site a % goes to the charity. Now where did I see that link