Assuming that the Internet connection there does not block VoIP traffic, he could make US calls from Gmail. This has nothing to do with OBi. If the IP address is a US address (on the base, it may be), or he connects through a US VPN, the calls will be free; otherwise Google charges $0.01/min.
If you have an OBi in the US, he could connect via OBiApp and use your GV account for free.
On restricted Internet connections, Skype will often work when systems such as the above do not, so it would be useful to have as a backup.
