Legal Question in Criminal Law in California

There is no such thing as consensual sex with a minor. By definition, only adults can consent to sex. Minors cannot. Sex with a willing minor is usually called statutory rape, though California law no longer uses that term. It is also possible to commit forcible rape on a minor, which is a more serious offense.

I'm not sure what you mean by "bring in" evidence. The girl's grandmother is not a party to the case and thus cannot introduce any evidence in court. But she can share evidence with the prosecutor and/or the defense attorney. If she does this far enough in advance of the trial, they may be able to introduce it.

Whether the document you describe will be admissible will depend upon many factors, including what it says and whether it is relevant to any disputed issues in the case. Without more information that's the best answer I can give you. The grandmother may want to consult with a lawyer about what she can and should do.