[PantsOnFire]

The ruling is award BB the main pot and button the side pot. As well, BB would get a warning or a penalty of some sort. Any compensation or departure from that ruling would simply be between those three players and their conscience.

In a casino, this may be easier but since you described this as happening a home game, it sure does have an impact as you seem to imply.

It doesn't seem to me that button would have folded if he didn't see that 5. If it had been me and this was a game with my buddies, I might have shipped half of the $400 I won from MP back to him.

This situation is not really as bad as it first seems. However, there could be a situation where it would be patently clear that one player benefitted. For example, if button had the second nuts like a K high flush and BB flashed the A, then button now knows for sure that he has MP beat. In a situation like that, as the host I might ask that button and MP take back their $400 or have BB ship his main pot to MP.

In a good home game, you can only hope that all players involved recognize the damage of such an action and find a fair way of spreading the money in the pot around.