Addiction is complicated and nasty. If she's a gambler too, even if she's not addicted the way he is, then she's got to have some really strong mechanisms in place keeping her from seeing his issues all along. It's quite possible that she IS addicted, but that she's just at a different place from him on the path towards the bottom. This sounds a lot like a younger me with alcohol and my alcoholic ex-husband (who hit bottom a few years after we split up).

I think that sadly, the friendship is at risk no matter what you do, because her mechanisms of denial may cause her to cut you off if you say anything to her OR if she feels you've been holding back. You've got to do the thing that you'll blame yourself for the least if everything goes to h*ll - the thing that you can best live with in your heart. It sounds like that's telling her.