Of course he loses. But, as I've said lots and lots before, it's all about value. If something is priced at 100/1, but you can assess the true odds to be 90/1 then in the long-run you will win.
One of his big things now is betting on football in the last five minutes. His reasoning is that people on Betfair who are on a winner with the finishing line in sight will be desparate to lay off and ensure that they aren't killed by a late goal. As a result, they will lay, say, 1.2 when the true odds should be 1.1. Harry is more than happy to accommodate them.