Right to return for full refund?

Hi all. My mother’s car packed it in so she bought a 2012 Toyota Prius from a local small garage. She’s not the brightest spark because didn’t notice that there was over 700km mileage on it. Anyway, fast forward to her second time driving it (the first being to drive it home from the garage), and a “check hybrid system” warning comes up on the dash with a big orange warning symbol, and the car would only crawl home at 30-40km/h. I called the garage being very polite and he was an absolute arsehole, making out like we were trying to scam him or something because my mother had paid him €1300 on the day and sent another €1400 by cheque in the post which he hadn’t received yet. We definitely sent it though so he’ll get that today or tomorrow we think. He hung up the phone. Anyway, anyone know if shes in her rights to get a full refund considering surely it’s reasonable to assume the fault was there or developing when she bought it? I know that’s very high mileage but surely this happening on the second drive is pushing it. Advice appreciated!