That doesn't make sense. Through a web browser he is being sent 2 channel PCM audio - so at best he can emulate 5.1 with the receiver, which sounds like garbage compared to true multi channel audio. DD is a digitally compressed 6 (5 speakers + subwoofer) channel source audio that requires something to decode the package on the other side. The computer isn't decoding DD, it passes it thru to the receiver. But if you aren't being sent DD in the first place, there is nothing to decode. Only the app has licensing for DD.
THX cert is a completely separate issue which has nothing to do with multi channel audio, and applies much more to the audio quality and not digital decoding of multi-channel audio.
http://www.technobuffalo.com/2010/01/12/does-thx-certification-matter/
Note that there are other versions of DD that support more channels - DD+ can support up to 7.1.
Also, if you are using windows 7 right now and it's activated, you should not have an issue upgrading as long as you use the accessibility tools (which, hey, who doesn't?).
https://www.microsoft.com/en-us/accessibility/windows10upgrade