iPhone stolen in Spain...how best to replace?

My daughter is studying abroad in Spain. Despite my warnings how to keep her belongings safe, her iphone was stolen. When she first arrived, she removed her SIM card and purchased a prepaid SIM card that she was using while she is in Spain. She still has the original SIM card (not sure if this matters?).

Because she will be living in Spain for 3+ months, she wants to buy another iPhone, but obviously needs to purchase it there. I called Apple but the rep had to do some research and basically said she could buy a phone at the local Apple store, but the warranty would only be valid in the country where she buys it so if it breaks after she returns to the US, repairs would only be covered if she returned it to Spain. I’m not sure whether that is accurate, but my real question is....will she be able to use that iPhone with our local carrier (Verizon) when she returns to the US? Will the SIM card from her original iPhone be compatible and if not, what does she need to do when she returns to be compatible with Verizon? I’ve tried to research this, but all search questions seem to only pull up answers for people using US purchased iPhones while traveling abroad, not the other way around.

She is planning to,go to the Apple store in Madrid tomorrow but what questions does she need to ask? From what I can tell, she will need to purchase an unlocked phone which I assume will have to be paid in full at purchase? Does Apple even offer a payment plan in this type of situation or is that only if upgrading an iPhone?

most importantly, I want to make sure that if she purchases the phone at the Apple store, it will be unlocked/compatible with Verizon when she returns to the US in a few months.

thanks for any advice you can offer.