This made me think a bit. Your sister will have her phone. Assuming it is GPS enabled, perhaps she should download area maps (for off line use if no data service)and use one of the free GPS programs to geotag important locations (home, work, other useful places).

Even if there is no cell service and all power is out, the satellites will still be working. And with the right program, it will give her a simple pointer of the correct direction from wherever she is.

The thing that concerns me about compasses is that it takes some skill and record keeping to keep on track from a known point to a desired point. In a post earthquake city, that may be more difficult due to blocked streets, and areas that must be avoided (fire, flood etc. etc.) Also, I don't know how all the steel in the buildings might affect the accuracy of a compass.

Sadly, I can't recommend such a program (I use one oriented toward my walking time/distance/speed), but I seem to remember a thread a year or so ago discussing such programs.

In any case, I agree a proper compass as suggested is to be preferred to a button compass.