I suppose it really depends on how well she treats me and how engaged she is in our marriage and building a life together.
Truth be told anyone whose had more than one relationship is only with their mate because things didn't work out with their ex.
Sometimes you're the one ending a relationship and sometimes it's them. Most people eventually remove their "rose colored glasses".
They accept the fact that in order for your (ex) to have been "the one" they would have to see (you) as being "the one". At the very least a "soul-mate" is someone who actually wants to be with you! (And vice versa). If someone doesn't want to be with you they're not the one!
Having said that if my wife were NOT engaged in building a life with me, was romantically distant, constantly reading old love letters and looking at pictures of her ex, or I found out she was (secretly) sending him love notes and expressing her true heart's desire is to be with him....I would set her free.
Clearly she has not "moved on" from her past relationship.
In your case I'm not 100% sure her soon to be husband is "in love" with her if his main motivation for proposing was because of (you).
Some guys hate the idea losing a woman so bad they're willing to propose to keep her. It's not unheard of for some women to give ultimatums to guys they've been in long-term relationships with.
"Marry me or else I'm out of here!":)