Assuming that the girl is of consenting age, of course it's legal. Having bi-polar disorder doesn't make you incapable of thinking. One of my best friends is bi-polar; she takes her meds regularly and you'd never know there was anything wrong with her.
There are no statutes or case law because the law does not take a position on the matter of who may sleep with who as long as they are both over the age of consent.

