No need to do anything.

If she's been unfaithful, she will receive her paybacks one day. It's only a matter of now or later.

Trust me b'coz my ex had been unfaithful to me, left me and end up she's been dumped by that guy after few months. =)

*peace*

Do not take revenge coz we're not God. We do not have the say on other's fate no matter how bad they treat us.