I managed to bypass the changed with:
function onMenuOpen(e) {
var uid = $(e.target).attr('data-uid');
var scheduler = $('#AfsprakenSchedule').data('kendoScheduler');
var event = scheduler.dataSource.getByUid(uid);
setMenuForEvent(event);
}
This doesn't select the appointment, but by keeping track of the current appointment with the right-click i'm able to act on the correct appointment.
Thanks for the suggestions