The adapter for your flat mount brakes will allow you to use a 160mm rotor on that bike. However, you will have to replace the 140mm rotor to her bike as the current one will no longer work. Is the convenience worth the cost of a new rotor? Also most wheel swaps require at least minor tuning of the shifting and perhaps rotor/caliper realignment so it's not as simple and fast as just popping in a new rear wheel.