Great idea! If you want to surprise your friend and plan a Donkey trip around the city, you can do it easily!
There are two options:
1. Book bikes and set your friend as one of the riders. In this case, your friend will receive an email informing them that you have invited him/her for a ride.
2. You can set your e-mail address for both bikes! You will be responsible for unlocking and locking your friend's bike, but your friend won't know about the trip until the last moment.
Have fun together!