He doesn't need a new group, just shifters. The only reason to get more than just shifters is to move to 11-speed, and that still doesn't require the whole group plus that's not Tiagra. Tiagra shifters is a viable option for sticking with 10-speed if they are available separately and dirt cheap. That would make sense, but personally I would move up to the 6800. This is a damn good excuse, and that is what we are all looking for, right?