What are you asking? What is your legal question? You cannot seek legal action against someone who simply doesn't want to have a relationship with you any longer.
Unjust enrichment is a principle of British law, not primarily American law and it has nothing whatsoever to do with relationships. Unjust enrichment deals with the obtaining property at the cost of another. For example, lets say I am not home and someone walking past grabs my weed eater off the porch and "sells" it to the neighbor down the street. I drive by my neighbors house and see him using my weed eater. Upon being notified that it is mine, he must return it. This is also the case even when the property isn't obtained through illegal means. This has nothing to do with a relationship ending.

What exactly is it you are trying to do? Sue someone for dumping you?