2nd Gen Question about legendary Pokemon in the Gold / Silver game

Hi,
I played "Gold" around 2000 on Game Boy color.
I have a question that has been bothering me for years. At the time, we discovered a lot of secrets, including previous-generation transfers and "duplication" of Pokemon using Glitch. The only Pokemon we could not get are celeby and Mew.
One day my friend just "got" them, he never said how. I passed them to me to complete the 251 Pokemon. Mew disappeared after a few days (a glitch side effect) .. and I was never answered the question of where he got them from ...
We live in a country where it is unlikely that an event took place in which such Pokemon are distributed.

I have tried to find out over time but there is no information about the possibility of getting these Pokemon officially.
The save of the game was deleted for me years ago so I have no proof or anything but that's the situation.

Does anyone know how to answer that?
Thanks

There were events for Celebi in Japan, the US, Europe, and Australia back in the day, but if you don't live in one of those places, my guess would be that your friend used cheats or glitches to get a Celebi. Couldn't have been a trade from Crystal since back then the GS Ball thing was only available in Japan until the VC re-release a couple of years ago.

The Mew could've possibly been transferred up to Gold/Silver from Red/Blue/Yellow via the Time Capsule thing, but that then leaves the question of how he got the Mew in the first place, but given how glitchy R/B/Y is, it could've been through glitches. That's probably not the only possibility though.

There was a glitch similar to Missingo in the first Gen which enabled a player to encounter MEW but it destroys the game save file. I have a yellow save file with over 70 hours of gameplay and the hall of fame is corrupt all because I wanted a Mew and did the glitch. Its a pretty cool Pokémon to train though.

i've watched videos of people glitching in Celebi specifically, but it was so convoluted and ultimately broke your game if you did anything wrong, so i doubt your friend did that.

like Nah said, 99% sure he found a way to hack them in or something. almost always the case when they wouldn't explain how they got it lol.

I don't know about your friend, but I've got a setup on my Silver cartridge and on my VC version Silver on my 3ds that allows me to essentially reprogram any aspect of the game I like, from changing the stats and gender of a Pokemon to changing their species, making them shiny or not, giving them any moves I like, etc. I've used this to get all kinds of interesting things, though I like to keep the movesets legal. It could easily be used to get literally anything you like, but I've never had any issue like the Pokemon disappearing.

Here is the video guide I used to set this up:
https://youtu.be/PsIb3OZaYAs

Mew you definitely can get if you have Red or Blue via a glitch. In Yellow it's a lot more difficult but I assume it's possible as well. I got a couple of them back in the day and was able to easily trade them to my Gold and Crystal version without repercussions.

Celebi is a lot harder to get and involves some glitching that can possibly cause some issues if you do it wrong. However if you got it traded and it worked fine there shouldn't be any problems.

Pokemon don't just disappear, they are simply data. And even though you can mess around with glitches and cause all kinds of weird behavior, the games themselves are actually surprisingly robust. I suspect you probably ran into some weird special case that only happens very rarely. Doesn't help that the save file doesn't exist anymore. That makes it harder to track down...