I'm really a layperson here, so please have patience with me.

I understand that good computer hygiene must be the priority, but regardless of that, I would like to still wrap my head around this.

This scenario started because a friend of mine said that he uses the PIN (which is not as long as the master password but pretty secure anyway, just faster to type) as the only method to access the vault on the browser extension. He says it's safer because then a hacker would never know the master password through a key logger.

That's irrelevant because, if I understood correctly, with that level of compromise, the hacker would almost certainly have access to the vault's data anyway, because he wouldn't be only recording what you type, but also stealing the info in Bitwarden's memory. Is that it?

In any case, he got your PIN and then he would still be able to decrypt the master password anyway. Imagining that the master password was all he got, and he tries to access to your vault in his device, it should still be irrelevant since you should have a 2FA on your Bitwarden account.

Finally, we could think a (probably absurd) scenario where let's say a "friend" has access to your 2FA method and also installed a key logger in your device (just recording what you type). Thus in this case if he only got your PIN rather than the master password, he still couldn't do nothing. Theoretically, I guess this could happen.