Don't be ridiculous, truth is a defence against Libel (written down is libel, not slander).

The onus is on the person who said the thing to prove it is true ("guilty until proven innocent," since it's a civil rather than criminal offence), but as long as he has documentation/emails it's all good.

Indeed, I would be astonished if they sued him for this, it would not end well for them.

Disclaimer: not a lawyer, but an election agent.