From nowhere, my friend got a notification that
com.google.android.trichromelibrary was downloading together with a progress bar.
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From Shared Libraries on Android which explains native code in Chrome on Android
- Android L & M need ChromeModernPublic.aab
- Android N, O & P need MonochromePublic.aab
- Android Q needs TrichromeChrome.aab + TrichromeLibrary.apk
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.
Edit: From Android Police
... Chrome no longer appeared an option in the WebView picker (found in Android's Developer Options). A Google engineer explained that Android 10 uses a new implementation called 'Trichrome':
"Chrome is no longer used as a WebView implementation in Q+. We've moved to a new model for sharing common code between Chrome and WebView (called "Trichrome") which gives the same benefits of reduced download and install size while having fewer weird special cases and bugs."