Private Calendar problem

Hello.

I set up a family calendar – on my Mac, and shared it with my girlfriend. the calendar showed up on her iPhone and on her Mac. so far so good.

She can add, view and edit events on the iPhone.

But on the Mac, events show up and the calendar shows up on the side menu, but is missing if you want to assign said calendar to an event. Checking the calendar preferences, the shared calendars do also not show up in the default calendar list. She can edit events I created, but cannot even selet the calendar to create new events.

To make things even stranger - on the Me.com website, she can select and create events in this calendar.

Any ideas how to treat Calendar on the Mac without deleting all her calendars?

(both machines MBPs late 2011, running Mountain Lion latest)

thank you,

Stefan.