One way that you could do this would be have a "Tracking Sing", either in a Process Variable or a SmartObject. So in the activity that has the client event, add an event before the client event that concatenates the destination users' name (Act Dest Inst --> USer --> Name). For this to work, you need to use the Create a Slot for each Destination and resolve Roles and Groups in the Destination Rule. So you will end up with a value like this:

K2:DomainUser1; K2:DomainUser2; K2:DomainUser3; K2:DomainUser4;

After the client event, add an event that removes the user's name from the list . I.e., let's say User2 completes (pseudo code):

UserNames = UserNames.Replace("K2:DomainUser2;", string.empty);

So when the event expires, the users in the string value have not completed the item.