It's a release 8, so do this: On Ext. 15, add a DSS button for Ext. 10, and a button for Feature 27 (Message Alert Notification). The feature 27 might have to be programmed from Ext. 10 through Centralized Telephone Programming.
Now your user at Ext. 15 comes back from lunch, presses the Message Alert Notification button, which lights up, and looks at the button for Ext. 10, whose light indicates the MWI status at 10. Press the Message Alert button again, light goes off, Ext. 15 and its buttons are normal again.