It is an example of "Social Exchange theory"
Social exchange theory is a theory that simply means that the social behavior that people exhibit is as a result of an exchange process.
According to the social exchange theory, that was developed by George, as individuals, we weigh the potential benefits and the risks that are associated with a social relationships.
In a situation whereby the risks outweigh the rewards, then individuals will end or abandon such relationship. This is the case exhibited by Bethany and Raquel in the question.
In conclusion, the answer is "Social Exchange theory"
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