How/where/how much will it cost to fix this dent?

[veevee]

Left the car with a female friend for a week, came back to a large dent in the rear quarter. "It'll pop out". Seething.

I've never had anything like this before, is it repairable? And how much will it cost? I see lots of cars with really ropey repairs to large dents, so I'm pretty concerned!