[CraigFL]

The real problem here is that a "sample of one" your car vs. their car really makes it hard to prove anything. There are so many variables like alignment, tire pressure, tires, road surface and corner weight that make testing difficult and expensive.
My test would put a device on the steering wheel to measure the force it takes to center the steering wheel, keeping the car going down the road straight. Before testing, you would need to try to measure the two cars according to the parameters I gave above.

After this kind of testing, you would have determined what is wrong and what needs to be fixed-- so then why not fix it and end the situation...

Good Luck!!