I had similar happen at her age, back before smartphones and before online check-in was as common (at least to me) and I had to print my boarding pass so I figured I'd do it all at the airport. There was a massive accident on the freeway so I was close to the deadline, and then a bus unloaded a huge number of seniors who didn't know how to use the automated check in machines. There were no in person lanes open for checking in until you had gone through the machine, and no staff to help at the machines, so I was frantically trying to help them to clear a machine for myself. As I was entering my info but before I finished checking in the time clicked over to too late and the system booted me out.

The differences? They refused to let me run for the flight, despite how much I begged. And it was a legacy airline.

I learned a valuable lesson about leaving plenty of time or knowing I was taking a gamble.