Ring the motor vehicle repair authority, they are like the department of fair trading, speak to the customer service person, most of them are former mechanic's, tell him/her exactly what you asked to be done/got done and tell him//her what you were quoted and what you are being charged and tell him the mechanic's business name and address and phone number. They will probably send out a dispute form, don't fill out straight away, tell your mechanic 'friend' of your intentions, most mechanics will back down and an agreement can be reached without having to lodge a dispute with the motor vehicle repair authority.
I had to do this when my engine rebuild went $1000 over the qouted price, and again when I payed for two new driveshafts and only got one