That zener doesn't make any sense (used the way it is). Zeners have relatively poor recovery time from reverse bias to forward bias due to their higher junction capacitance. The are also relatively expensive compared to a diode. It may improve the drop-out time of the relay, over a junction diode, but not by much. If a zener is going to be used it should be across the transistor, where it goes into zener avalanche very quickly when the transistor turns off.