How to get crash logs of Testflight external testers on Xcode or iTunes connect?

Is there any ways to know what's wrong with someone's app. It is working on everyone else's device but this person.

The person tried removing the previous version and installing the new one, but it still does not work. I suspect that one of the files that I place in the documents folder might still be there... but the format of this file has changed since...

Apple has a technote about this named Debugging Deployed iOS Apps.

Also, if your tester syncs up their iPhone with a Macintosh or PC, iTunes also helpfully moves logs into predictable places where they can be retrieved from.