Just explaining it to her won’t be enough, I think the Op should be active in trying to get her to see that she engaging in gambling is going to ruin her if not properly managed and he shouldn’t just stop at that; he should get her to do something else instead of gamble.

He should also be careful not to force her too much because that would only make her become more secretive and would lead to her still gambling but won’t want to show it to the Op because she knows that he’d probably force her to do something else instead. It’s actually not an easy thing to achieve but I think with the right guidance you should be able to get her to leave gambling.