Is it illegal to extend mortgage repayment on behalf of others?

Not illegal. It's not illegal to postpone the mortgage repayment on behalf of others. Regular small loan companies are only allowed to do business in the local area if they postpone the mortgage repayment on behalf of others. They should handle it in person, and shall not charge pre-loan fees or withhold interest. If there is no agreement or unclear agreement on the loan term, the borrower may return it at any time, and the lender may urge the borrower to return it within a reasonable period.