This is hard to troubleshoot being it's someone else's computer, probably at another location. If this tester is part of your development team you could try to write some simple test programs to clearly isolate the problem. Such as a very simple program that loads and displays a png image.
This is where a major developer using a more traditional platform differs greatly from hobbyist/indie DB developers. You probably don't have the resources to buy a bunch of different computer configurations to test on in-house. Also DB doesn't do any error handling. Usually people talk about the fact games crash, but in this case you could seriously use a detailed error/crash message.
You might want to look into getting a plugin that offers error handling/trapping. I heard STYX can trap errors but I'm not sure exactly what it's like or if it would specifically help here. However some sort of error code or message system would really help you here, especially since you are communicating with testers.
