Now that we understand the situation, you might want to visit
this thread, where the subject of how to enter the US on an immigrant visa is discussed. But briefly, she doesn't fill out an I-94, she does fill out the customs declaration and checks the "resident" box. Which line to wait in depends somewhat on the airport and may vary from time to time in the same airport. Best bet is to ask an officer who is directing people to lines, but if you don't see someone to ask, the "visitors" line generally works. There's a good chance the "citizen/resident" line may work as well.
BTW, this thread is far off topic in this removal of conditions forum. This forum is about filing the I-751 after you've had conditional LPR status for around 1 year and 9 months. POE topics are normally discussed in the "Moving here and your new life in America" forum.