This has happened with a number of altcoins recently. Basically the only solution was for them to patch the client and hard fork.
Exactly, the altcoins are a nice micro-environment for testing extreme scenarios such as this.
The outcome has been that some altcoins have implemented much faster difficulty-retargeting (such as every 100 blocks instead of 2000 for example). This greatly mitigates the negative effects of a drop in hashrate, as the difficulty is fixed more quickly.