He told his Daddy that she was sleeping around with black players on the team (zero evidence we are aware of). Also that she was ugly, but had big tits. And had a potty mouth (another charge that suffered under scrutiny). I don't think its yeti trying to find a racial angle here. Peyton was leveraging it to trash her reputation.
Also, how does Favre's case, one with no tie to racial animus that I am aware of, shed any light on the entirely different nature of Manning's conduct?