Phone seized? Declined entry?

Wow. FWIW, US border control agents can inspect people and property without a warrant when you enter the US- or actually if you read the statute, up to 100 miles from your port of entry! This includes passwords and access to content on any device you're carrying, a right that rarely is utilized unless you're photographing the arrivals area (which is not permitted) or otherwise are a person of interest.

But if she's just a young Brit minding her own business, perhaps she encountered a martinet who spitefully insisted on accessing her device and refused entry when she declined.

Also worth knowing that not all of the constitutional protections accorded to US citizens apply to aliens except by conventional courtesy. This traditional courtesy Brits once expected is being eroded because of concerns about terrorism and the hell to pay if authorities try to focus on nations that they think are riskier. But this access to device rule applies to US citizens as well.

Once you know more, please keep us informed as to what happened.