"My ex is crazy" is player-speak. It's a pre-emptive strike against the possibility that the ex calls or comes around, crying, with a plea or demand for what's rightfully hers -- child support, the money he borrowed, the stuff she left at his place, or just an explanation for why he dumped her. The objective is that Ms. Right Now does not, even for a minute, take Ms. Ex and her pain seriously.
It could very well be that Claire *is* delusional, that she did cheat and that the baby isn't Brad's. But my own experience in the Player's Club is telling me that "Brad's a cad" is a far more likely scenario. I hope that Claire has a good lawyer. If you feel the need to say *anything* Brad-related to Claire, encourage her to do that, and refrain from passing along his cruel comments.