You don't mention the type of paint, which would be a good thing to know. I assume latex, because I can't imagine why someone would be washing out an enamel paint brush in anything but paint thinner..

Preliminary Googling suggests sunflowers as the go to solution for detoxing soil contaminated with heavy metals. Might be worth looking into what all it's supposed to 'clean'.

While I'll grant you it's not something you're going to want to water your garden with, there are some paints out there that probably won't harm your garden in the slightest, and trying to 'fix the problem' might make it worse. The type is going to be really important into getting an educated answer (if there is one, I'm not sure if there is much you can do).

Chill... unless he washed out a paint tray and saturated roller. The amount of paint left on from a brush is pretty minor. no need to redo your work.