It does not matter what a mechanic told you, regardless of his experience or reputation. There are only three possible causes for what you describe, and you can determine with a fair amount of accuracy which is the case.

1. The wheel is not dished to the outer locknuts on the hub. Whether one can correct that by moving spacers is determined by how much space is available between the outer cog and the inner surface of the right dropout, and less commonly by how far outboard the derailleur can move. If you reverse the wheel as explained previously, with it mounted in the same position in the droopouts, and it shifts in the other direction then the dish is the problem - period. If it's in the same position then it is a frame/rear triangle problem.

2. The rear triangle is not aligned properly. You do not need a mechanic to determine that either. Just run a taut string from the outside of one dropout, around the head tube, and then back to the outside of the other dropout. Measure the gap between the seat tube and the string on both sides - the distances should be equal to each other. If the rear triangle is off to the left that side will of course show a larger gap.

3. The dropouts are out of alignment. This is the one problem that is somewhat more difficult to determine at home, but you can get a good idea by putting the bike upside-down on a stand or with a friend holding it, and sight along both sides of the front wheel toward the rear wheel. You should be able to see the same amount of wheel on both sides and neither should be tilted in relation to the other when you look over the top of the wheels.

I should note that the only way the dropouts could be out of alignment in a way that would cause the problem would be either an original frame defect or damage to a seat stay. I don't see any indication of damage, so it would have to be a frame defect - very unlikely to have not been detected and addressed previously to you owning the bike.