Upvote:2
She needs to clear immigration when her flight arrives and then claim bags to clear customs. Immigration counters are staffed ONLY when there are arriving flights, so coming through later may not even be possible. Plus staff at the airport will flush all the arriving passengers out of the jetway halls into the immigration area to make sure everyone is accounted for. Unlike international airports in other parts of the world, there is no sterile transit area at American airports for international travelers, all arriving passengers must officially enter the USA when their flight lands.
If she had her boarding pass, perhaps she could have dropped her bags at recheck after clearing customs and re-entered the departure lounge. But I am not familiar enough with Logan Airport to know if the airside areas are open all night long. Plus TSA may prohibit her from entering the sterile departure area that far in advance of her departure time.
The airline is not responsible for providing accommodations on a long layover. Some airlines do provide meals and/or lodging, but they do it simply as a PR benefit and it tends to be airlines trying to carve a niche for themselves. There are no laws requiring it.