What if nothing happens when I double click the avchd folder?

My friend's computer can't open these files in Quicktime (by double-clicking them) but if I right click and "open with VLC" they will play on his computer, perfectly.

If your friend is running macOS High Sierra and you are receiving a "Can't open" modal message, then this is a normal consequence of having a codec package installed that conflicts with built-in High Sierra AC3 audio support. Suspect Apple now includes a sub-set of the latest DolbyDigital software package previously adopted for iOS and tvOS devices embedded in the latest macOS. Unfortunately, the new embedded support conflicts with third-party codecs/packages previously installed by users. Since AVCHD content contains stereo/2-channel AC3 audio, any attempt to open such content creates a system conflict when High Sierra can't decide which AC3 software support to use. If this is the problem, then simply remove *or "turn off") the third-party support software to allow High Sierra to function normally. Don't forget to "reboot" the system after removing/turning off codecs to reinitialize the QT/QL codec support lists. (I.e., QL changes don't take effect until the system is rebooted and QTX support changes do not take effect until the app is closed and reopened.)

If the problem is something else, then you need to provide additional, more specific information regarding the issue.

He's on OSX 10.12.6. What other information do you need?

In that case, Profile, Level, Color Depth, Frame Rate, use of any non-standard Profile/Level feature settings, etc. or a short MTS sample file for analysis. What system is friend using (i.e., hardware specs)? QTX or QT7 Player on friend's system? Basically need to determine if file averaged/instantaneous settings are supported by the QT AVFoundation embedded in the macOS or QT7 embed. (I.e., the recording device appears to have an impressive set of recording features and we don't know which features/settings were used to make the recordings and/or if friend's system can handle them.)

And a sample mts clip

00001.MTS - Google Drive

Thanks for posting the sample MTS file. Here is a fuller rendering of the file specs:

As you can see, the file appears to be a standard AVCHD/Blu-ray quality level encode. (I.e., High Profile@Level 4.2 8-bit 1080p60 file encode—with specs well within High@L4.2 2048x1080p@60fps 62.5Mbps max range limits.) Your friend might try playing the file in a BD media player to see if that helps but assuming his platform hardware is otherwise capable of playing the media content, then the only other issue I can think of is a possible hardware or software conflict at work here. File also plays fine at full resolution on my 5K monitor. Had been worried that you were making 4K recordings which might be having some instantaneous "out of bounds" bit or frame rate issues.

I didn't mention that he was able to play it on VLC on his system, and other similar media plays on that system so there's something really Quicktime-specific that's froopy.

Sorry! Forgot you had mentioned that in the last paragraph of your initial post. However, the VLC open source codec package is not as "standards" conscious as QTX which won't even try to play a file if the system thinks a file is not standards compliant. While all indicated settings appear to be standards compliant, there are a number of specific features not included in the stats above that aren't compatible with the High Profile (HiP) as shown below:

Unfortunately, I have no way to evaluate whether or not any of these represent a potential issue with your sample file but since the file plays on both of our systems, I discounted this as a likely problem source for your friend's platform. Do, however, agree that the problem is QTX specific regardless of its source on your friend's system.

Best of luck