Assuming he wants to be married to her and isn't indifferent, the calling his bluff would be by actually moving out/kicking him out (on a temporary basis) or otherwise demonstrating how important an issue it is to her.
If he really is indifferent, then she'd be best off out of a relationship with him.
If he isn't indifferent, then it should be a wake-up call and he can prove himself. (If he is feigning indifference to get her to cling on, it's a worrying emotional manipulation).
It depends whether the weed-smoking is a deal-breaker for her, of course.
I think the chances are high that unless she puts her foot down hard about this, he'll just repeat the pattern he's already exhibited: saying he's stopped but continuing secretly, she catches him out but nothing happens except he says he'll stop, and so it goes on. Could easily become a pattern in other parts of their life together.
It's the lying that's a problem, imo, and he's already told her the only reason he's 'stopping' is because she's found out, not because he wants to stop, not because of the money it costs, not because of anything else. IME people don't stop habits they enjoy without an inner drive to do so.