Correction: Boost and Cricket both use the Sprint CDMA network.
If he's bringing his own phone from a different country he'll need a GSM/HSPDA network. Globally, LTE should be the single standard once 4G is on worldwide rollout, but it won't be there for years. Maximum interoperability requires GSM. I've spent way too long looking at mobile phone standards in the last couple of weeks.
"No matter where you go, there you are..." -- Buckaroo Banzai