If warranted, help ensure a smooth transition. But don't "kill yourself" to do so. And... once you are out, be cautious of requests for further support. (Some might even ensnare you, legally. Ex-coworkers should not be sharing significant/proprietary internal information with you or asking you for help with same.)
Of course, there is the real world and maintaining friendships / not being a jerk. But keep the above in mind.