Or so dad ... and maybe you ... believe.
Obviously the two of them were doing something dad did not know about, so there is some reason to believe they might still be so engaged.
Well, no law in CA requires him to make the report. However, medical personnel, counselors, teachers, and others ARE mandated reporters. One day it may very well come out that this happened. By not reporting it to the police, Mr. Wonderful may remain out there taking advantage of other minors. Oh, by the way, what you describe appears to be a FELONY by Mr. Wonderful, so it is likely he will continue this activity if not stopped cold in his tracks. Eventually, he'll get caught but not until he knocks some poor girl up or spreads some STDs. If you believe that he will keep it in his pants after this, you don't know teenage boys very well.
But it would corroborate the allegation. If she says they had sex, and he affirms it in conversational texts or saved voice messages (neither of which are likely to have been saved by any cell phone company), then that makes a pretty compelling case. A record of the call or text - by itself - would not serve to prove the case, only to show the communication.
While charges may not be likely, it is a good idea for the police to have a record of this so that they can keep an eye on the kid. If it truly is a one time event, no real harm done. If it is part of a pattern of events that is ongoing, then it might serve to stop a predator.
He probably can NOT get in trouble for NOT reporting it. Unless he was concealing the crime to protect the suspect, then I cannot think of a crime that would occur by his failure to report it. However, if it can be argued that he has condoned or supported his daughter's unlawful sexual activity, he could be charged with a crime.