re: OT Airline Experts: miss departing flight, can you still board return flight?

What happened to her is the "flat tire" flight change. Most airlines/agents know this stuff happens: over sleeping, car trouble, interstate closed, etc. and will be moderately flexible in rebooking with availability.

What happened with her return flight was simply an IT/agent error when they rebooked her on the flight and didn't reinstate the original return flight.