I do this all the time domestically. Two things: 1) only carry-on luggage, and 2) do not identify your frequent flyer account when you book.
If she were doing it on an outbound flight, I’d also add, 3) book two one-way trips, not a round trip. Otherwise the airline will cancel the return.
I suppose there could be risk of rerouting to a different layover stop due to weather/whatever. But in that case she could go back to the Boston plan.
If i were a passenger who was making a connection, especially one with the complications of customs clearance, I would appreciate that the airline was looking for me/holding the doors for the connecting flight. And if I were trying to get on standby, I would rather the flight didn’t go with the empty seat.
With that reference point, I would probably tell the airline I wasn’t getting on the Boston flight but didn’t expect a refund. And I think in the world we live in, it’d be fine to say you weren’t feeling well.
“With that reference point, I would probably tell the airline I wasn’t getting on the Boston flight but didn’t expect a refund.”
If not taking the last leg of the flight would entitle you to a refund. It may make the ticket more expensive. A lot of times direct flights into a major airport are more expensive than connecting ones.
My husband used to fly a lot. Oftentimes, his meetings would end early, and he would try to fly standby. Here is what he experienced. Airlines end boarding and shut the gate doors a few minutes before departure… if a ticketed passenger was not at the gate and boarding by that cut off time, the gate agents would grab a standby to fill that spot. On some occasions, the passenger would run in just when my husband was getting his standby boarding pass… and no, the airline employees would not let the passenger reclaim their seat. My point? If there is a stanby, the seat will not fly empty.
Many shut the doors 10 minutes prior. My husband has been on time for a flight…but missed that 10 minute window. Someone else got his seat!
We flew back to NYC from Amsterdam via Dublin last summer. Clearing customs with Global Entry went smoothly, but the Dublin airport is “cozy” with limited seating and a couple of options for food so you may want to bring snacks if you have a long layover. We ended up resorting to vending machines bc the lines for food were so long.
One time my husband went to get a snack when it was close to boarding time. I told him to hurry. When they started boarding, I tried calling him but he didn’t answer. So I went ahead and got on the plane. Not long after, a woman started settling into his seat. I told him she didn’t have the right seat, but she did! She was a flight attendant and they gave her the seat when DH didn’t show up in time. Oh, well, she was very nice and I made a new Facebook friend. 
Thanks for the info. We have a 13-hour layover, sigh, because the ticket was a lot cheaper. It’s from 10 pm until 11 am the next day. I hope we can find a place to camp out. We will bring snacks!
You may want to check into a hotel near the airport. It’s not really comfortable. My husband is an Aer Lingus frequent flyer and even though he uses their lounge, it has limited hours, so he’s spent many hours in the waiting area. He’s pretty low maintenance, but even he complains.
13 hours is typically plenty of time to go through any needed immigration / customs / preclearance there.
OK, thanks so much for the advice. I will see what I can find.
I assume he didn’t have checked bag under his name? If he did, they would be calling his name with their loudspeaker and wouldn’t give his seat away.
I don’t recall if he did or not. Must not have!
So, he missed the flight and you stayed on it anyway?
That’s awesome. 
I suspect in this era/aftermath of covid, no airline will make you fly if you feel unwell. Or charge you extra for not boarding a flight sick. I understand the cost consequences of open jaw tickets as well, but at that point - fully paid last leg - I think you’re safe. Of course, you could just grab your bag and go too!
Oh, yes! I warned him! He’s always late while I’m always on time. But to be fair, I’m very messy and he’s very neat. 
DH agreed it makes sense to get a hotel so I booked one that has a 24-hour free shuttle. We don’t want to be any more exhausted when we get home than we have to! Thanks again for the recommendation.
Yes, you are probably right. The last time I flew, way back when in the '90s, they had no problem with rules like these. Now? They likely would let you skip the last part if you aren’t feeling well. At least, I sure hope they would.
Hurray! Have a wonderful trip.