Applying the Strawman will most likely work if done properly.
The goal is to plant seeds of doubt in the girl's mind about the relationship. And the way to do it is to raise her expectations about the relationship to unreasonable extents.
This is done by pointing out and exaggarating good qualities and even making the bad ones look like good.
Every girl, ever, will have SOME doubts about her boyfriend. Little or much it doesn't really matter because if you find them and build a strawman it will end the relationship in a trainwreck, unless the girl loves him to death. Which is not the case 99% of the time.
If you know the guy is outgoing you can point it out by saying:
"wow he must be the party. I bet he's taking you to new exciting places every single day"
"You look so happy with this guy, I bet he's carrying you on his palms and provide you with everything you ever wanted"
"You seem so confident in this relationship, like not a single doubt. This guy must be the coolest person you've ever met"
"You're shining. I can tell that out of all the people you've ever met, you have the deepest connection with him"
Things like this. As you can see no guy can ever stand for standards like that, so if these lines are delivered in the correct moment they will initiate a tsunami of doubt and will just destroy the relationship. Usually guys are oblivious about this and will have no idea why their girlfriend is acting weird, and will fuck it up further. It's good if he knows that you two still hold contact because that can make him jealous. Jealous=Busted.
Just grab onto every opportunity to raise doubts and make sure the guy has an opportunity to fuck it up every minute.
You know it when some girl wants to find her poor friendzoned chump a girl? And she starts telling her friends that "He's an amazing guy" Lists a shitton of great qualities, and without wanting it, she just cockblocks him because he'll never be as good as she told so. It's pretty much the same like that, but this time, the guy happens to be the boyfriend of your target and you're doing the cockblock.
Regarding the meeting. Just be cool. It's a perfect opportunity. You meet him, stay cool, make your wife laugh a couple of times, and with some luck it will click something within him. And after the meeting you can start praising the dude in front of your wife like he's superman. It's really important with the strawman to calibrate though. So be careful with this shit, because if done wrong it can come off like you're just jealous. It can help a lot if you have other women in your life on the side and your wife knows about that.
Yeah I know that everything about this is fucked up, but I can't really think of anything else.
Just wussing around and fucking her won't make her leave him, because it's comfortable for her. He provides for her and you fuck her, and everything is fine. She needs to realize that their relationship is not going to be a fairy tale and also that you have other options. This combined attack on the relationship will wreck it, especially since you mentioned he's a crack-head, and they're not usually the people with the strongest character and longest temper and best problem solvers. So everything is playing into your hands.
If only you could somehow make your wife find out about the drugs on her own, and then later when she tells you, you can tell her:
"I knew it, but didn't want to tell you, because you looked so perfect together, I didn't want to ruin what is clearly the love of your life." And that's gonna be end of relationship.