Like most things, the dose makes the poison. A little Red Dead is probably great, too much probably bad. Gambling, food, sleep all fall into similar U shaped curves.

Who are we to say how much is the right amount for this person? To play 6,000 hours of this game he must have a reason. Perhaps he's particularly passionate about it, perhaps there is someone in his life he loves to play it with.

I think that argument works a lot better if we're talking about a consumption level in the 10-100s of hours, along the lines of average for this title and most others like it:

https://howlongtobeat.com/game/27100

6000 hours is multiple orders of magnitude beyond that point, so it feels like a point where it's a lot safer to reach for an absolute judgment that it really is "too much".

It's a multiplayer game

Fair, and I know that's a different ballgame if you're going to compare with Destiny 2 or WoW or League or something. But nonetheless, I think there's value in at least having some measure of calibration for what is reasonable and healthy.

But you don't know his life. Why judge from afar? I'm not saying it's normal or recommendable for the average person. I'm just saying, we don't know.