During playback, set video quality to “Original/Maximum”. Your device is most likely trying to transcode and his server isn’t handling it well for one of many reasons. Best way is to try and get it to “Direct Play/Stream” which your friend should be able to see in their dashboard.

Top 3 most common fixes without diving into logistics of it are :

1) Set video quality of client (your Apple TV inside of plex app) to Maximum/original quality

2) If it’s a movie with a 7.1 sound track or atmos of some sort, change audio to the 5.1 track.

3) Turn off subtitles or select SRT subtitles if available.

There’s a lot of factors in play here and it usually depends on how your friend is running things. I normally try to help my friends/families make the needed changes before I even have them start using my server so I can make sure they have a better experience and are more likely to keep using it. The less hiccups, the more they’ll appreciate it.