Wow, I could never do this for the simple fact I would never be able to date a chick in this country again if I pulled a stunt like this, I'd have to move overseas because it would be all over the news.
That said, I have no respect for somebody who promises monogamy and then goes out and cheats(In public) even after they discovered the engagement ring and realizing the other person plans on proposing, and not even telling them about it beforehand.
Now just before any of the ladies chime in with what a jerk this guy is, I am pretty certain that had this situation been the other way around, all the women in the world and on this forum would be praising her for it and encouraging her.
With that in mind, I dont feel so bad about laughing at this.
Revenge is a dish best served cold.