Were there any other representatives of your company on board?
While I don't have any experience with this sort of thing in a corporate flight department environment, if you strip that away, and take it on its face what they did-
They were abusing one of your employees. They were taking damaging company equipment. They were diplaying a total lack of respect for your company, it's assets and employees. If this occured not in the cabin of the company jet, but in a board room would it be appropriate? Or if they treated a female executive like that at a golf outing? Or even the receptionist? Hell no! So why is it ok to do it to a company FA?
I don't know what level employees they are with your client, but at the very least I'm sure THEIR bosses would like to know how they respect other companies.
A good manager looks out for those underneath them, and their department. They abused your FA and reduced her to tears. I think you owe it to her to bring this incident to your boss's attention; leave it up to them if they want to persue this further. If she's afraid of losing her job, then cover for her. Tell them she didn't want to go forward with it, but that you felt it was your duty to bring it to their attention. They may make the decision that the client is too important to piss off, or they may be so disgusted with them that they'll make it known that this sort of behavior is not appropriate.
But let THEM make that call. Just my $0.25 ($0.02, adjusted for inflation)