Are you asking about manually deleting files from your hdd/ssd or deleting the caches under the Data tab in the game, or asking if you should do this before/after an update? This thread has gone 2 ways with answers so far.
If I had to guess I’d say your friend is telling you to manually delete files? Don’t delete anything manually in ThisPc unless it’s the phantom 8 gig cache file that apparently gets created in the default install location (if you install in the non-default location).
As far as removing the cache/deleting the cache in the Data tab in-game that’s going to be up to you. As crunch said you’ll get many opinions on it but basically you’ll probably see no great gains either way. I would keep at least 8.
And finally, before/after an update, yes you should delete your caches as those files may have been changed.