Two Close Friends where one of them is is about to or is already screwing some chick/dude, and without the other (3rd party) person knowing, they swap with each other.

The "Bae and Switch"

A pick me is a woman that is willing to do anything for male approval. She will embarrass or throw other women under the bus to achieve this goal. The unfortunate thing about a pick me is usually the men they are trying to seek approval from are of poor quality and treat women badly, leaving little real benefit for the pick me.