Pre-owned iphone with faulty battery

My mom (an elderly woman) bought an Iphone SE she was not told is pre-owned and the phone battery doesn't last up to 5 hours even if its not used. When returning the phone we are told it should be returned within 7days of which on the receipt it is not written but says return within 30days. They took advantage of my old mom and the fact that she doesn't know anything about Iphones. This is disappoint