I always go with the "the charmed becomes your best friend - but he does not suddenly consider his old friends his enemies". So, how would the charmed character react if two of his best friends start fighting?
I don't know about anyone else, but "help one side to kill the other" is not my usual answer.
Coincidentally, such ruling also made getting charmed much, much easier in play, and more fun for the charmed PC, who does not have to attack the other PCs, but can play peacemaker, and such. It also emphasises the difference between brute-force ("Attack them, my min... err, friend!") and more subtle ways ("Of course I don't want to hurt anyone, they there want to hurt me, I am not harming anyone, please protect me!").
I don't know about anyone else, but "help one side to kill the other" is not my usual answer.
Coincidentally, such ruling also made getting charmed much, much easier in play, and more fun for the charmed PC, who does not have to attack the other PCs, but can play peacemaker, and such. It also emphasises the difference between brute-force ("Attack them, my min... err, friend!") and more subtle ways ("Of course I don't want to hurt anyone, they there want to hurt me, I am not harming anyone, please protect me!").