If it's deemed by the council to be a one person flat they might not allow a second person to live there- may be considered a sort of subletting. So you need to check. I think TTT means it might affect your benefits - assuming you are on benefits. May also affect council tax element of the rent if rent includes council tax
If his home is subject to a mortgage, he may need to advise his lender as most mortgage loans are granted for "owner occupation" and he may become in breach the terms of the loan. He may also need to advise whoever insures the property and/or contents. Both mortgagee and Insurers may wish to impose conditions (eg additional interest/premium charge if they approve of the letting).