Google FI works pretty well for international travel. It doesn't cost any extra in many countries.

I've travelled a lot; NA, Europe, Asia and Oz and used data / mostly WiFi to wassap call / Skype.

I live in London, data only will be fine.

I just buy a sim card there, it's pretty cheap with data, just swap it into his current phone.

I bet he'd get away with spending $25.

Yes, this is the best bet. Just make sure the phone is unlocked. General/about/carrier lock for an iPhone

I use Google Fi also, no extra charges for data, no matter what country you're in.

Internal cell calls did cost extra thou, like 10 cents per minute. I usually used data apps to call back to states, but used cellular for the odd locally calls I might have to do.

That said, if you want to stick to just data, that's fine also as he'll probably have wifi access most of time also.

I used Whatsapp usually to call back to states, but other data enable apps work also.

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Fi is especially great now that the "unlimited" plan doesn't kick the throttling in until 50GB. It's also always been great on giving you the same level of coverage (at least up to 4G, don't have a 5G phone to test if it goes higher) abroad as you get locally. The T-Mobile "free" international is fine in a pinch, but being throttled to 128/256kbps (at best) is pretty painful for real usage. The Fi calling rates are also pretty cheap for most countries if you're on WiFi (one to two cents a minute). They also don't seem to be too particular about extended times abroad which some other carriers don't love.

That said, if you're doing something short term, you're probably fine with data only and getting WhatsApp (it's super popular in Europe/outside the US anyways). You can do messaging and voice and video calls from it.

Thanks for all of the replies. I found an Orange SIM on Amazon, looks like it has mostly data and works in UK. So I should have that prior to him leaving, but if it doesn't come in on time I'll have him stop by a store to pick one up. He's installing Google Voice now, so we'll have that all setup.