Phone warranty

My sister bought a phone at revibe, after a few weeks the phone had a green stripe on it, she then contacted revibe since it had a warranty, the phone was then shipped to revibe to be inspected, after the inspection, revibe claimed the phone had other damages when it came to them which they wont fix unless she pays a hefty amount, even when she sent them evidence of how the phone arrived to them they kept replying her with AI emails until they declared to throw away the phone coz she wouldn't pay the hefty amount they wanted to ship back her phone or fix it