It sure is a huge amount of money, but it's really not that impossible though. Believe it or not a small minority of people can get really lucky and hit the jackpot with low cap altcoins; the Shiba Inu situation being a really good example. $15k -> $650k is uncommon but nowhere near impossible.
Yes, I'm not saying it's impossible, I'm just skeptical about these kind of stories I read in the forums.
In any case, if he has hit the jackpot, that means that he was gambling, and not investing, so, he'd better get a lot of information before investing again, and that means not just reading some opinions in a forum. Alsol, he should invest only what he can lose.
If he has more than half a million dollars that he has obtained by gambling and he is not a rich person, I would take at least half of it and save it to buy a house, for example, or to finish paying the mortgage, if he has one, and then with the rest, to invest wisely.
pooya87 has made a good analysis. The OP, assuming what he says is true, reminds me of the guy who comes to the casino, hits the jackpot on the first spin and continues playing until he ends up losing everything.