The iPhone 6 from T-Mobile is unlocked. Factory unlocked for that matter. I got an iPhone 6 T-Mobile and I was able to swap SIM cards in between ATT and T-Mobile.
The iPhone 6 has every LTE band needed for you to receive 4G/LTE if it is available in your country. If you are either in North America, Central America or Latin America the T-Mobile version will work for sure. Model A1549. But if you are somewhere in Europe you may need a different version, Make sure you check the LTE bands if you have any questions:
https://www.apple.com/iPhone/LTE#mn_p
Verizon models are Factory unlocked as well and you will be able to use a CDMA carrier. You can buy them full price at the Verizon store. If your friend has a SSN and $20 for activation to spare you should be up and running.
In case he doesn't, the T-Mobile model is factory unlocked and ready to be used internationally.