Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
If she had been staying at a hotel in Jersey City, I probably would have tried leaving the luggage behind at the hotel, gone out to EWR, and picked up a car from there -- assuming that the rental rates for an EWR pickup/return were reasonable. Then, stop at the hotel to pick up the luggage either before or after the trip into Manhattan.
Unfortunately, weekend car rentals in and around the NYC metropolitan area tend to be pricey -- especially in the Summer.