I have a Mobal international cell phone (mobal.com), which works in most countries. It comes with a SIM card good almost everywhere. You get a UK phone number, so if you're traveling in the UK, calls are free. It's a bit more expensive than buying individual SIM cards in different countries, but the extra expense is worth not having the hassle.
If you have an unlocked GSM US phone, the same company will sell you a world SIM card.
Your friend can also check with his/her US phone provider to check on temporarily adding an international calling plan while traveling.