Remote Silencer (Beta) description
This app and it's widget allows remote volume muting and unmuting of multiple android devices.
Commands are send through Google cloud messaging service so both devices must be online but it has advantage that app doesn't need to run on background. Application of commands is non acknowledged for privacy and lesser footprint (so no one can check if you are online or you are just ignoring him).
App is still in Beta so report bugs and post feature requests at http://junkcode.net/forum/viewforum.php?f=6