I want to call out my roommate's fiancee in front of a crowd for her manipulation and selfishness.
Why would you do that to your roommate's fiancee?
She's manipulating my roommate and isn't genuinely interested in him
You're right that your roommate's fiancee should not be behaving this way, but he has clearly made his choice already.  If you have a problem with her behavior, this is something that should be discussed privately between the two (or three) of you, and not shouted out in front of a crowd.
I'm going to do it anyways because I don't care what people think.
I really think you should reconsider. Your idea could cause a lot of tension between all of you.