Why can't I join a friend's world on Minecraft PE?
Solution 1:
You must be connected to the same network to play together, unless you use a third-party app.
There's a third-party app called Multiplayer for Minecraft PE that allows you to host a server and connect to other networks. It's definitely available for iOS, but I'm not sure about Android or other app stores.
The other option is to connect to the same wi-fi network that the other person is on. This will require you to be at the same physical location as him.