I don't know about calling OM into the office... I would present this as an office matter and that the management is entitled to know about it.. You don't want to look like you are asking him to solve your problems...
Take as much evidence in there as you can...
Just tell him he is entitled to know what his staff are using work time to do and how this is likely affecting office moral... Not to mention they are likley putting their middle finger up at office policies as well...
I think just from a workplace standpoint you can make a case that management is entitled to know...
If that happens to put a dent in their dirty little secret and add a monsoon of stress on them... That's just GRAVY...
