In your case, Handbrake produced an MP4 with specs your friend's Mac player can't handle but your WMP can. Most of us with varied collections of video files from multiple sources consider the built-in players that come pre-loaded with the computer OS as stop-gap or placeholder apps: we install VLC or MPC-HC or PotPlayer, etc as our primary video players because theres much wider file compatibility and formatting tolerances vs the default OS players. Its a roll of the dice with either platform. It does play some MP4 files my Mac video player won't, but on a surprising number of occasions the reverse happens and I've been able to play MP4 files on my (really old) Mac players that the latest Windows 7 WMP chokes on. Windows Media Player on my updated Windows 7 laptops remains fairly lame, with inconsistent compatibility.

Maybe MS provides better regular program updates which keep these things up to date, whereas perhaps MAC OS does not. Handbrake can spit out some pretty peculiar files sometimes, even if it seems like you asked it for a totally common standard format. VLC will usually play anything you can throw at it, no matter how off-spec or oddly formatted. The VLC player recommended above is the universal player almost everyone on every platform relies on, even if they also install additional players they like better. Things may have changed with the very latest MacOS and Win10 updates, but I haven't checked. And neither the Windows nor Mac 'native' players will play MKV at all (which is just bizarre, considering how long thats been more popular than MP4 at this point). Sometimes it will easily play video files that popular third-party players have trouble with, but generally both the Mac and Windows default players have trouble with anything not encoded to a narrow, typically older MP4 standard. Just as with Windows, the 'native' Mac video player can be picky. So the default video player on a Mac won't play H264 MP4? Really? Depends on the age of the Mac and specific OS install its running on.

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