I'm with MachIVshooter, JohnM and nickn10 (and others, just having three in a row is remarkable)
Bullies shouldn't be tolerated, and you should consider who might be next in line for a dose of bullying. Perhaps the next person to get targeted will be a 70yo in poor health, or an encumbered (expecting or young kids in tow) woman ... after all, if some scumbag is hanging around a parking lot, he/she can match up nicer cars with easy marks. MachIVshooter and myself* are unlikely to encounter these parasites if the parasitic plan is working.
I'm not suggesting vigilante justice, but rolling over for a bully just teaches them that they have a lucrative, easy, and safe job. People who make their living by threatening violence need an occasional reminder that they're in a dangerous profession and should rethink their career choices.
So, yes, I'd be likely to come to the aid of some geezer being harassed in a lot, if the circumstances were as described and obvious. If they were less obvious, I would at least get po-po involved.
I don't know what's up with this "go get the manager" plan ... seriously, is the retail store manager going to don armor and get on a white horse? A retail manager may be a good guy, but is he going to drop everything and run out to the lot and run off a ruffian for you? I want to shop/eat/do business/etc in these places, I've been going to the wrong parking lots, obviously!
* there are really two BFD's, there's the 6'3" guy who can reach out and make a mugger into a broken ragdoll, and there's the fat guy with a gimp knee ... I can't be sure which I'm projecting at any given moment, so I have a "surprise!" just in case I'm accidentially being too nice or am otherwise being the fat guy with a limp ... or I encounter a particularly dedicated scumbag, or otherwise get selected as a victim