IE 9 Compatibility Issue?
Using Jamroom
The rule of thumb is current version minus one. So IE11 and IE10. The problem you run into as a developer is how much time and effort do you spend making sure your software supports older versions for a handful of people. You might tell him to try Firefox or Chrome if he has it on his system. Vista however died out a long time ago and who knows if any of the current browsers actually support that OS.