I have a friend that is new to riding mtb and he had a budget of 2000 dollars. There is a bike that had a 3x8 drivetrain and there's a bike that has a 1x9 drivetrain. He says that the 3x8 drivetrain has more gears = better but I've told him a million times 1x9 is better because it is simpler and more clearance but he still says no but if I make it more specific he might actually buy the 1x bike but I don't know how to make it more specific. Please help me make him actually buy it by replying of how the pros and cons are and maybe a little specific too. Thanks!

I'm not sure I disagree with your friend. 1x works quite well for MTB, but if you're new to the sport and aren't sure what exactly you'll need out of the bike, it might not be a bad idea to err toward a richer sprawl of ratios.

Why do you want him to buy the 1x9?

And what's that about clearance?
Do you spend much time log jumping?

Clearance is only ever important for very special kinds of riding.

3x9 has a wider range.
Admittedly, I don't use the granny much. Snow riding, mountain touring.
I could probably be quite content on a 2x9.
But 1x9, no.
I'd either lose speed on the easy stretches - or riding to the ride spots - or have to put up with fairly big steps between gears.
Wouldn't be particularly nice for my kind of riding.

Simpler - not much.
You lose the left shifter, that's about it.
Many 1x bikes need some sort of chain guide anyhow, which is comparable to the front derailer.

And are the bikes otherwise comparable?
Brakes, forks, wheels, fit, size?

You found a $2000 bike with a 3x9? What decade is this?

Haha, no. It was 600 dollars cheaper and still had decent parts.

He said he's gonna ride the bike until he will get to expert. I rode 3x8 before and I noticed the chain falls off front if I skid down a rocky decent. With a 1x I found just use a chain guide to stop the chain from falling off (front).

Whether a bike will or won't drop a chain under various conditions depends on a lot of factors, and what happened with one non-1x drivetrain may or may not happen with another. And since your friend is just getting into things, dealing with an occasional drop isn't necessarily a big deal. A bit of sketchy drivetrain behavior, if it occurs, may still be preferable to a less versatile drivetrain.

3x8 on a current-model bike says to me that the parts spec is low-end. That bears out in the $600 price difference that you mention. What if you broaden your field of view to the rest of the components on the respective bikes? What is the extra $600 buying your friend? A better fork, maybe? Better brakes? Hydro brakes? What's the bigger picture?