Mechanics do this on occasion to women. I try to handle all automotive things so my wife doesn't have to deal with this problem, but I travel for work several times a year and it's only a matter of time until the car breaks down when I'm on the other side of the country. If my wife ever gets threatened like your gf I'll be pissed beyond belief, and they'll know it.

In any event, if I were you I'd ask for an itemized breakdown if one is available. Then I'd talk to the manager and explain your side of things (calmly and respectfully), and explain that you don't believe the charges are reasonable. I'd also mention the way your gf was treated. Basically try to negotiate a more reasonable bill. You'll likely strike out on all counts, but it may make you feel better.

Also, once all is said and done, report them to the BBB and any state consumer fraud agency you can find. Most state attorney general's offices have consumer fraud units, for example, and some are quite aggressive. If nothing else you can create a headache for them.