Does anyone have experience getting reimbursed for fare difference fees for missed flights on Swiss?

My parents were booked on lx 23 New York to Geneva and showed up to airport just 75 minutes before their flight. The Swiss queue was so long that by the time they reached the counter they were not allowed to check in (they were not allowed to skip the queue and being elderly they don’t know about online check-in). Swiss declared them a no-show and canceled their return flight even with them at the airport before take off. They went to the Swiss agent desk to rebook and were told that they could not be rebooked because they had reserved on Expedia. They were told Swiss could not help at all. They called the travel agent who said they needed to call Swiss.

A few hours later I call and Swiss tells me that there is nothing that can be done because I didn’t notify them before the flight took off. I explained that they did speak to an agent before the flight departed and were told nothing could be done. I finally get Swiss to rebook and they charge change ticket fees for going and return plus fare differences for both flights. For the two of them this was $1600. they had even paid $400 for extra legroom on their original flights which wasn’t available on their rebooked flighths and Swiss refused to refund that as well.

My question is this - can an airline cancel your return flight and declare you a no-show if you show up at the airport before flight departure but after check-in closes?

ive missed flights before and generally get rebooked if there is availability on the next flight. I’ve never paid a fee anywhere near what Swiss charged. My parents are in their 80s and sometimes they have difficulty with travel. They are almost never late but combine NYC traffic, with my father’s heart condition and a long line and this sort of thing can happen. Even if Swiss can technically throw the book at them, I don’t understand why they weren’t more helpful in this situation. The Swiss rep at he airport didn’t inform them of their rights or who to contact or what to do. They simply said hey couldn’t do anything for them. Then when I tried to follow up with Swiss later I was told that it was my fault for waiting so long to notify them.

What is is my best option for getting some of the fees reimbursed?