I'm in the same situation currently, trying to fix and make changes to an iOS app that was initially done by a contractor.
First thing you need to understand is it is not an easy job for any developer to take over some else's mess. Every developer uses his now coding method and his own choice of libraries/frameworks to develop the app. It is never easy for any developer to take over some else's project, no matter how good the first developer was. A non-coder should know that same app can be developed in many different ways using different frameworks and stuff, each developer does in a way which is most comfortable for him. Every developer feels his methodology is best and other's code is always a mess :) Unless you have specified in detail exactly what frameworks to use and coding structure to use (for example use auto-layout or use storyboard....), it will be difficult for new developer, you need to understand this. It will take a while before the new guy can fix a simple thing in app, cause he has to go thru the entire code and project to figure out stuff.
The best you can ask is the old developer to document all the Classes in iOS project, comment every method in the code, document high level app flow (similar to storyboard). What 3rd party frameworks used, why were those chosen. What iOS frameworks were used (example MapKit, CoreLocation,...)
Using this information you can advertise that you are looking for example: "iOS coder to finish a project that does blah blah, that uses these frameworks...., storyboard interface, and auto-layout and blah blah"