You need the dSYM directory for the build your customer has.
When you build a release build, Xamarin.iOS will automatically create a YourApp.app.dSYM directory next to your YourApp.app directory. When you send a build to your customer you must save this directory somewhere on your machine (the next time you build your app the dSYM directory will be overwritten) [1]
Later, when you get the crash report from your customer, you can run the symbolicatecrash
script from Apple to convert memory addresses into function names and line numbers. [2]
[1] The Build->Archive command does this automatically.
[2] The exact location varies between Xcode releases, but it's somewhere inside your Xcode.app.