Ah, so the new version with the XML format has a "pptx" extension, whereas old PowerPoint saves as "ppt".

That explains a lot. The professor had Office pre-'07 installed. Your sister-in-law sent him a pptx file. Professor double-clicks on file. Windows pops up and says "I don't know what this file is". Professor is ignorant or lazy enough to not bother trying to open PowerPoint and selecting "File->Open". Professor gives up and says "it's not compatible".

This is actually a pretty typical example of two people unfamiliar with computers trying to do something vaguely complicated with computers. Neither one understands what the problem is and the one with the problem doesn't offer any sort of information in trying to solve it.

I think that the biggest problem here is the rejection without explanation. If you ask someone for something and then reject it without specifying why, you're at fault. Imagine a case where you order an album online. Then the album shows up and it's on vinyl. But you only have a CD player. You don't send it back and say "it doesn't work". You send it back and say "I don't have a record player".

Of course, I may be making facts up about this case.