Trichromelibrary was suddenly downloading from Google Play Store From nowhere, my friend got a notification that com.google.android.trichromelibrary was downloading together with a progress bar.

Why?
From Shared Libraries on Android which explains
native code in Chrome on Android


Android L &amp; M need ChromeModernPublic.aab
Android N, O &amp; P need MonochromePublic.aab
Android Q needs TrichromeChrome.aab + TrichromeLibrary.apk


(Emphasis added)
Further down the page it explains what these packages do (in developer speak). So, it's perfectly safe and legit, as it is a part of Android Chrome - nothing to be worried about.