[atrainpsu]

The only thing that was affected by the flashing of the card was whether the button called the 2 allins on the river. MP would have lost the main pot either way. To give him part of the main pot seems absurd.

It seems reasonable for BB to either pay the button the amount that he would have won in the side pot ($400) or forfeit the main pot to the button, whichever was less.

I'm guessing the main pot was over $400, so I think it should have gone down like this.
MP loses his $100 on the river and whatever else he put in previously. Button loses the $100 on the river and whatever else he put in previously. BB wins the main pot, which he still deserves to win. Now instead of MP paying button the $400 for the side pot, BB has to pay that instead.

This solution was the best case scenario for MP (if button had called him anyway, he would have gone broke), the best case scenario for button (if he had called MP anyway, he would have won $400 in the side pot), and BB pays a penalty for screwing up the action.