If he has a smart phone then an interface would allow him to plug bass into interface, then into the phone and use a wide range of apps made just for this purpose. Amplitube is a great app with loads of sounds in it, and he can import music into the app to play along with it for practice.
Unless there's no way to recharge the phone while camping. In that case it might not be a great idea.
Depending on whether the phone is iOS or Android there are quite a few options for the interface.
But if money's not an issue, then I'd also vote for the Boss Waza Air headphone set up. It's very clever!