No expert here, but I feel like straight firing would open the company up to lawsuits (and keep in mind, these RIFs do not happen without Legal's buy-in, guidance, and consult), especially given the obvious focus on longtime/tenured employees.
If they roll everyone up into a RIF, with younger/newer/non-class-protected employees within, they have a safety net on discrimination lawsuits. In a recent RIF that primarily focused on black/latino and/or older and/or disabled associates within one org, those with otherwise good cases were turned away by attorneys because a single unprotected class associate was retained. Of the ~10 associates laid off that day, precisely one was not in a protected class. And of the ~8 they DIDN'T lay off, only one was (in a protected class). That one associate saved them.
One of the RIF'd was straight up told by an attorney, after seeing the evidence the associate had compiled over the prior two years and agreeing it was damning, "you have a case here, but I can't take it, because this was a reduction in force and they kept another black person, and one who is older than you. It's nearly impossible to argue discrimination in a RIF, especially if those impacted are not all in the same protected class."
Anyway, the point is that if they had simply fired her, versus RIF'ing her, that attorney would've probably jumped at the chance of an easy settlement.