Do they need other gear, helmet, gloves, shoes (advantage with flat pedals for starting out is you can use old trainers initially). You can get an ok new bike for £500, eg something on sale from last year’s model range. Big brands like Specialised Rockhopper or Giant and Cube’s are good value (direct import to bike shop so one less middle man).
As noted above if you search around (eg on clasiffieds here) you can buy a used bike which would have cost around £1,000 plus but you have to know what you’re looking for and be prepared to travel to look at it.
I got started on a £350 Specialised Hardrock, to be honest I should have spent more as it was V-braked with a rubbish fork which I subsequently changed, £500 you’ll get a much better bike which will provide a great introduction to the sport. Have fun.