Respectfully ask the owner to demonstrate that the damage could not have happened in one of their repair stands. This is easy to do. If the clamp arms can not be made to line up with the dents they will have made their case. But of course the owner could claim it was done by another shop with the same stand.

It is really impossible to do this kind of damage yourself with out a tool of some sort. The bike falling over, something hitting it, etc: the damage would not look like this. I have had steel frames dented by accident, this is not the result of damage by use.

The suggestion of a vacuum: not possible from an engineering perspective.

For the owner to defer to the mechanics is NOT OK. The owner is the owner, the mechanics are employees, the owner needs to ultimately take responsibility for what happens in the shop. The owner shouldn't browbeat the mechanics in front of you, but to shrug and say 'well, the mechanics say otherwise' is avoiding the problem.