I am generally the type to sit back and let things happen but doesn't this guy deserve to know before he destroys his life.
If you know the guy and have a real relationship with him, yes, talk to him.
If you don't know him other than knowing who he is about to marry... . I don't see much chance of a good outcome.
You describe your husband's ex as a snake. You don't go around kicking snakes. And you can bet she wouldn't be happy with somebody "warning" her future husband away if she knew about it.
And if he doesn't know and trust you, why would he listen to you instead of the woman he's about to marry?