I don't like to pass so many cool places when I ride long distances so usually don't move that far in between camping for a day or two. But I am sure you have had days where you hit 70 even 80 miles and could keep right on riding. I average between 10 and 15 mph fully loaded because I try and conserve as much power as possible. If she's smart she's is carrying 3 batteries so she doesn't have to destroy a pair of them on this ride alone. Ya can't fully charge and fully deplete a lithium battery and be shocked when you get it's minimum life (# of recharge cycles). If she is the type person to methodically plot every move, every turn, even stop for the night and possible power at daily stops like cafes she won't have a problem in my opinion. This ain't her 1st rodeo it seems.Unless you are extremely hardcore 12+ hours a day on a bike, day in and day out for a month is going to be a major sufferfest. Even on an electric bike.
Long Beach, CA to Brooklyn, NY is a little over 3000 miles, which is reasonably doable in two months at 50 miles per day. Or you might kick up the mileage a bit and have a zero day every now and then. So if she carried two spare batteries and maybe an extra charger she'd certainly be fine. Anyway at 50-60 miles per day your only looking at (at most) 5-6 hours of saddle time per day. Which leaves plenty of time to stop and sniff the flowers, enjoy a beer, or evangelize about e-bikes.
My own $0.02 is that having more than three batteries total, without major charging infrastructure, is unlikely to be helpful for most long-distance e-bikers.