So you are basicly undercutting this guy. He maybe asks to be paid upfront because he's had problems getting the money after he has done work in the past and as for the gambling then it's no ones business but his what he does with his money.
Before slating him the customers must have agreed to the price and his terms of pre payment so if there not happy with his service and want to change cleaners then that's up to the customer, but it could be they want a cheaper cleaner.