If she has been checked in then the UA flight DCS at the Gate would show that he has been checked in for the flight. It would also show that she has bags. The positive baggage reconciliation is actually the other way about. Bags do not travel if the passenger misses the flight. It is not that the passenger does not travel if the baggage misses the flight. In this case there would be a lot of passengers not traveling on each flight or certainly many flights.
In my experience the 30 mins is usually to correlate the number of passengers and the actual boarded figures. Also if any passenger did not turn up at the Gate then the baggage will be offloaded and hence the aircraft can still keep to the push back slot.
This is the first time I have heard is also but UA may have new set of rules now.