The jury instrux are supposed to be only about the law, not the facts. E.g., the instrux tell the jurors what factual elements of the offense the prosecution is supposed to prove, and then say: If you [jurors] believe, based on what you've heard in court, that the defendant did [whatever the elements are], you should find the defendant guilty. If the judge were to instruct the jurors not to believe a witness or how much to believe, the prosecutor should leap to his/her feet and move for a mistrial.
IOW, it's (supposed to be) solely the jurors' decision whether to believe a witness and to consider facts like delay in reporting. This is one reason there's often a psychologist-type as an expert for the prosecutor or plaintiff in CSA cases--to persuade the jury that puzzling things, like delay in reporting and a child's changing from denial of CSA to admission, do not make the complaining witness less credible in these cases.
It's been a while since I read the Sandusky trial transcripts, but I don't recall anything as outrageous in the jury instructions as the judge instructing the jurors on the value of expert testimony about delay in reporting. Of course, counsel are free to rag on the opponent's witnesses (at the proper time!), but once the court has qualified an expert to testify, the expert may testify on the state of knowledge in his/her field. Then it's the jury's sole prerogative and duty to assess the value of the testimony.
I think the judge was correct to refuse to tell the jury that they shouldn't believe a prosecution witness. Amendola was free to do so during cross or in his closing statement, but the judge isn't supposed to take sides.
At this point, let's just hope that the PA Supreme Court doesn't vacate the conviction and remand the case for more proceedings! Who wants to endure THAT? Peace!
John (Esq., but I don't play one on the Web)