Yes you would. But the actual proof is probably far more damaging than what she has wrote. Considering she doesn't have to name him to be sued for defamation, he would most definitely be suing her if she cannot prove her claims as this will end up costing him money. That he can prove in court.

So frankly the proof you need is her not being sued and him keeping quiet, or giving out a non-statement which doesn't call her a liar, but claims his recollection is different.

This is going to be something that will most definitely cost him $X in the coming months alone, not to mention in the next few years.

Considering the backlash already happening, he will sue if he has cause.