What is the test for restitution in the context of an individual's claim?

"An individual is required to make restitution if he or she is unjustly enriched at the expense of another. [Citations.] A person is enriched if the person receives a benefit at another's expense. [Citation.] Benefit means any type of advantage. [Citations.] [] The fact that one person benefits another is not, by itself, sufficient to require restitution. The person receiving the benefit is required to make restitution only if the circumstances are such that, as between the two individuals, it is unjust for the person to retain it. [Citation.]" (First Nationwide Savings v. Perry (1992) 11 Cal.App.4th 1657, 16621663.)