Your mechanic can't be trying too hard to fix the problem.
He suspects the ABS? Can't he disable it, and see what happens?
Other things that might be looked for are brake pads that are odd to the other side, brake pads (either side) that have been soaked with oil or brake fluid but not changed, disk that may have been machined without doing both. Then slide pins, rubber hose, etc.if not already checked.
Finally, check whether both tyres are the same, and in near enough equal condition. Should you find the wheel is always locking at close to the same point of revolution, check for a flat spot on the tyre, and measure the thickness of the disc at multiple points around the circumference, to ensure it is an even thickness. Someone may have machined just one side as a "quick" repair.
I'm not saying do these checks in this order, obviously you would do them easiest and cheapest first.
