Originally Posted by Rosque
Then it's much simpler than represented just break up.
Unlike sexual orientation, relationship orientation (monogamy, polyamory, etc) is a product of worldview which has been developed over time. As with any worldview it can (and arguably should) be adjusted with the addition of new information; at least there is no harm in doing a little soul searching to see if there is the possibility of growth.
I'm certainly not in favor of a person denying their nature for the sake of someone else, but I AM in favor of being open to new ideas and information. Just because someone has never considered polyamory before doesn't necessarily mean that it isn't right for them... maybe they just need to consider giving it a shot.