Anyone (other than private householders) operating a CCTV security system is obliged to provide a copy of any material showing a particular person upon receipt of a written request from that person. The company is entitled to make an administrative charge but such a charge cannot exceed £10.
Enter the name of the supermarket in the 'Name' field here and click 'Search Register':
http://www.ico.gov.uk/ESDWebPages/search.asp
Then click on the relevant company (e.g. 'Tesco Stores Limited' or 'Asda Stores Limited). Get your brother to send a written request for a copy of the CCTV footage to the address shown, marking his letter 'For the attention of the Data Controller'. He should either enquire about what payment is required or, simpler, enclose a cheque or Postal Order for £10 (because that's almost certainly be what the charge will be anyway).
Once your brother has the footage he can then decide whether it's worth consulting a solicitor.
Chris