And therein lies the challenge. 18 is the age of consent in CA. Even if she was a willing participant, she cannot consent. The law does not recognize her as someone who can give consent. So it is still statutory rape.
Perhaps in another state it would have been a different matter.
Personally, in situations like this I always wonder... what if they had just run into each other while on vacation in a different country where it was ok.... Could he have been prosecuted upon his return?