He can still use core. Find someone he can trust, and get a copy of the blockchain on a usb stick. Then he can use free WiFi to sync his wallet. A pruning node with no signature info ( when SegWit arrives) will save on disk space.
Maybe he cant even afford 5 GB monthly, so he simply cannot use full node. But thats fine, full node cannot be guaranteed to work for everyone, there are even people who just have available few hundred MB monthly bandwidth!