Ah right, you didn't mention that in the OP...
Okay.. This is a tough one. You need to work out the Device and Android version. Are they running a stock ROM, if not, then tough (mostly).
Then find out exactly what they did to produce the error and can they reproduce it?
You could possibly supply the User with a debug version of the App, so that they could try and replicate it with more debug on the App. Your call of course.
Have you looked and looked at the code, to try and see if you can pinpoint the precise location and then work out if there is some possible slim reason for the error being thrown and try and code it out.
Of course if it's only one User, then possibly leave it...? vs. thousands or more. In the error info. is there any extra info. offered?