What evidence do a prosecutor need to convict on a statutory case?

What if the 15 year old girl is merely upset about the breakup and just wants to hurt her 19 year old boyfriend? He says it didn't happen, she says it did. She says it happened once. He says it never happened at all. they broke up because he found out she lied about her age. Can this 19 year old boy be prosecuted? and convicted on he say/she say?