Bumped from Business to Economy Refund?

Hi, all! My mom recently flew from O’Hare to Heathrow. She is one of those people who has never splurged on herself for anything (she’s been a psychiatric nurse for 41 years), but because she’s had several surgeries (knee and hip replacements, plus other things), I convinced her to get herself a business class ticket round trip, so she could be more comfortable. Wouldn’t you know it, the flight before the long haul Transatlantic flat was delayed, and she got bumped from business to the main cabin on the way over. Does anyone have advice on who I can get in touch with to get that leg of her trip refunded the ticket cost difference? Thank you for any and all help!