If the REP is ultimately denied, she can be out up to 180 days per journey. If denied, she needs to immediately return to the U.S.
If REP is ultimately approved, she is not subject to the 180 day rule. She must return to the U.S. before REP expires.
There are plenty who leave the U.S. thinking it will be for less than 180 days. Then something happens, such as border closure, personal medical issues, family medical issues, etc. And then they stay too long. Of the few that attempt a return, most are permitted as returning residents. But it seems 1 percent get unlucky and get an NTA, especially in cases where CBP knows the arriving alien cannot easily adjust status.