Academia

Was your friend currently employed when they lost their visa?

Not my friend. You're confusing me with someone else.

While timing from the original comment isn't clear, the phrasing of "That mental health diagnosis was enough for the Japanese government to cancel her visa" suggests that the visa getting cancelled came first (and loss of employment was due to no longer having a valid visa etc).

This is landing permission. This is you entering the country before the actual visa (residence card) is issued.

Ah, good point. So on the one hand it seems odd that it'd be a requirement to enter the country but not to stay, but on the other hand many rights are often won on technicalities like this one.

That being said... from https://www.japaneselawtranslation.go.jp/en/laws/view/1934/en#je_ch4sc1at9

Article 22-4(1) Where any of the following facts are found with respect to a foreign national residing in Japan under a status of residence listed in the left-hand column of Appended Table I or Appended Table II ... the Minister of Justice may revoke the foreign national's status of residence
(i) The foreign national has received, by deceit or other wrongful means, a seal of verification for landing ... on the consideration that he/she does not fall under any of the items of Article 5, paragraph (1).

With Article 5, paragraph (1) being the landing procedures already referenced in my earlier comment. (You can also see the Appended Tables I and II here, https://www.japaneselawtranslation.go.jp/en/laws/view/1934/en#je_apxt1 but these basically cover almost every visa imaginable status.)

Keep in mind that I don't understand the exact meaning of "wrongful means" here and I'm certainly not an expert in Japanese law - so I'm just assuming that it means "obtained it incorrectly" aka there was an unintentional mistake or unintentional omission (in contrast to "deceit" which is defined as requiring intent).

Basically though, if I'm understanding correctly, this means that the law in Japan allows revoking the visa/status if it's later determined that granting the status during landing should not have been done.

So it might have been enough if, after somehow learning about the hospital stay (perhaps the friend of the other commenter told immigration in case the "brief" stay ended up being more than three months - which from "brief", it probably wasn't, but perhaps the friend might have been trying to be helpful to immigration here and also forgot to double check with a lawyer first), that Japan Immigration determined that the depression was a condition that existed before the landing happened and thus should have been reported (even if the friend didn't know about it beforehand). One can imagine an even harsher scenario - where the friend was falsely accused of knowing about the depression before coming to Japan and deliberately hiding it (which would then qualify as "deceit").

I'm guessing and speculating too much here. The original tale remains plausible to me, but to really understand it, it seems like more details would be necessary (which the original commenter might not even know, as it happened to a friend).

if they lost their job, their employer is obligated to inform immigration within 10 days of this happening

Not doubting you, but just curious, as I wasn't able to find a source regarding this. In fact I found something else, https://www.japan-guide.com/forum/quereadisplay.html?0%2059619 , which suggests the opposite is true (that employers in Japan rarely inform immigration) - but that one is also 15 years old and might be significantly out of date.

and if they do not find employment within three months, then they could be depleted

"depleted" is an odd word here - I guess you mean having their visa/status cancelled or revoked?

I i did find other sources ( https://www.japan-guide.com/forum/quereadisplay.html?0%20132389 & https://www.japan-guide.com/forum/quereadisplay.html?0%20171354 & https://www.daijob.com/en/guide/expat-essentials/important-procedures-for-foreigners-living-in-japan-2-troubleshooting/ ) confirming this.