In iTunes, with their device tethered, if the user control-clicks on the name for their device, they will be able to select "Reset Warnings". Afterwards, a dialog box will pop up when the user next Syncs their device asking if it's OK to send data to Apple. If they click OK, then iTunes will upload all the crash logs from their device to Apple's servers.
Afterwards, you should be able to find your crash logs in iTunes Connect, and download them yourself. New crash logs will appear if your app still has problems and the user Syncs again, all automagically.