Airport Extreme: How to share a hard drive over the guest network?
The most up-to-date version of AirPort Utility is 6.3.2.

According to this Apple Release Notes of AirPort Utility 6.2, you can extend guest networks as a part of the update.

The most current AirPort Utility is 6.3.2, which is available through software update. I however is able to find 6.3.1 in this Apple Release Notes.

This will solve your problem easily by letting yourself use the main network, and your neighbor using the extended guest network.