If she's taking her mobile phone, which might be a good idea in case of delays / hiccups with her flights, make sure she knows what data services cost when you roam out of the US, or if she's anything like my teenager you might as well be buying the airplane she comes home on.

Thanks for mentioning this. She also will want to keep the recharger with her, with a U.K. adapter. If there is a delay on either end, she might need to recharge it.

Not to mention the passports again but I recommend that they stay together in the non-EU line.