This person is asking for a $30k raise to do the baseline of what's expected of them: their job. It's your job to know what needs to be done and assert that it needs to be done.

They didn't make $500k, but saved $500k from a mistake that the company themselves made. Why does that deserve a raise? I've seen people implement features confined to a single file that made over $1m/year but they didn't ask for a $30k raise.

I could see a one time bonus or stock gift and maybe a more modest raise next year if they stick around.

My overall point is this: If you want to make the big bucks, you can go make your own company, but be prepared to take on the mountain of risk that comes along with it. Otherwise, a tech salary with guaranteed income ain't so bad.