Can you use an iphone in Europe?

I'm buying an iphone 5s for my cousin in Europe and I'm just wondering if there's a way he would be able to use it? Do I just have to buy it from t-mobile or AT&T because they have the SIM cards? I don't want to buy something that he can't use.

Only GSM iPhones sold by AT&T, T-Mobile and Apple in the USA use the mobile phone technology found in Europe and many countries around the globe. Verizon's CDMA iPhone 5 and 4S add GSM capability so they will also work in Europe. (Verizon's older CDMA version of the iPhone 4 will not work in Europe or anywhere else that uses the GSM mobile system.) All GSM mobile phones around the world, including the AT&T iPhone, use a SIM card that CDMA cell phones do not have.

T-Mobile (the one you are thinking of) & AT&T don't operate in Europe. You'd have to buy an unlocked GSM model, which only Apple Stores sell.