I believe you ought tell him since the pursuit of money via gambling is always disillusioning. He has been losing in years so why would he suddenly have the odds to turn his way? Such a better option is that he concentrates on developing wealth out of something more consistent and sustainable.
Ideally its something he can push forward to tell him, but in some situations, we may have to consider the condition attached before taking any necessary step towards that, because some people can easily pick offense when you're telling them the truth for their own benefits.
That is why we cant force people against their wish, even though we also need to take our own role, however, we are expected to be cautioned to what we say and the way we present them, so that we don't stand to ruin the relationship and make them feels bad and disappointed about us, it has happened before that some got into trouble by what they advised as the other partner picked offense in it.