I had a head gasket go, on a Ford Escort, last year. My (excellent) local garage charged me £487 (inc VAT) for the repair.
However I was delivering car parts for a Ford dealership at the time, so I was calling on lots of garages each day. Out of curiosity (and to check that my local garage was charging a fair price), I asked quite a few places what they would charge for the job. The cheapest figure that anyone else gave me was £500 + VAT = £600. Most places suggested that somewhere between £600 and £800 (+ VAT, making £720 to £960 in total) was a more likely figure.
If your friend gets a £500 bill, she'll probably be getting a good deal - but she might get a significantly bigger bill.
Chris