When my MIL (elderly, no status, wheel-chair assisted) went to select a one-way ticket last week, the seat map showed two open seats in coach. When she got to the purchase screen, the seats were gone and her ticket says "assign at gate". I told her "it happens".

She has an assigned seat for the second leg of her journey.

Currently, she's #4 (out of 4) on the standby list. It says there are 12 seats still open (6 hours before departure) which I assume are passengers who have yet to check-in.

I fly this route a lot and know they go regularly 100% full.

What's Delta's obligation if she doesn't get a seat assignment? There are three more flights on this route later today, but each of those would cause her to miss her connection on the 2nd segment. There's only one later flight on the route of her 2nd segment.

Any tips for making this easier on her?