So we have to distinguish between the civil case and a potential criminal case, in the first instance. Her civil suit is based on allegations of multiple forcible rapes and not statutory rapes. And complaints don't establish anything--they are just allegations.
As for the possibility of a potential criminal case based upon statutory rape, I don't know what defenses are available in California...I have NO idea. Her representations could be a defense, his age could be a defense, etc. The age of consent is 18, as you say, though.
I don't think there is an issue of fact as to whether some form of sex was had (assuming the kit reveals his DNA). But as above, I don't know whether there are obvious defenses that would militate against prosecution.