Sensibility and health hazards of reflowing computer PCBs in a domestic oven
They are almost certainly not in any danger.  Not any more than the fact that they handled the board and possibly ate something before washing their hands.  Direct transfer probabilities are much higher than a double outgassing transfer, first to oven and from oven to food.  In the oven of course you have the potential for outgassing, which then coats out on the inside of the oven.  I doubt that there is a lot of this.  On top of it, people typically preheat the oven which would spike the temperature up again beyond this "reflow" temp and this would liberate the bad stuff (if it was there) and carry it out of the oven.  i.e. it would be baked out.

Boards are relatively clean, lead is not very volatile, there are VOC present but once heated they dissipate etc.  you wouldn't want to use an oven that is exclusively used for this, but once in a while is most likely very very safe.

In fact the oven elements when new outgas on first use, after cleaning the oven there are chemicals left on surfaces that outgas.  Those would be the best comparables and more concerning than the PCB.