Dynamo - Reversing the rotation of the shaft
Sorry, your friend is correct.

A permanent magnet dynamo is the same as a permanent magnet motor. If the motor polarity is reversed the motor spins in reverse. Similarly the EMF generated by the dynamo will reverse with rotation reversal.



Figure 1. A really simple motor/dynamo. Source S-Cool.

The brushes aren't providing rectification of the supply - they're rectifying the windings. Notice that current can flow either direction but that the coil's orientation doesn't matter as the brushes correct it.