Don’t meddle, have faith in retribution
When the person you least expect to do you dirty pulls this kind of a stunt, it can be tempting to go all Hammurabi's Code on them and start trolling Facebook for an ex-girl of his (or friend of hers) that would be down for a little blow-for-blow (see what we did there?) retaliation.
The prospect of trailing each of their names through the mud and tarnishing their reputations will be equally enticing, as will diabolical meddling to ensure their relationship's speedy demise.
Don't waste your time on revenge or lame attempts at lashing out. Instead, maintain your squeaky clean karma (a clear conscience is godliness, or something like that) and rejoice in the knowledge that being a shitty friend is worse than having one.