This is really sad. Bullies in the workplace are vile. However, your DH doesn't have to give this awful man any power over him. I get he had a terrible experience, but this is an adult workplace where your DH is allowing some moron who doesn't even work there an enormous amount of power over him. He can choose not to.
If he is so anxious he has absolutely no resilience or ability to feel in his own power and have boundaries around what he will accept from a visitor at work, then this something he needs professional help with.
Objectively, DH has a perfect opportunity to show that he will not be bullied ever again,by anyone. If this visitor to DHs' work tries to bully an actual member of staff at that workplace, how will he get a job at that workplace?
DH and/or yourself just need to stay calm, nothing might even happen, the thought of things are often much worse than the reality. Catatsrophising won't help,or change anything, it just makes everything worse and is self sabotage .
If this bully even ends up working there,and that's a big if, things have changed and bully will be the newbie, and DH has to instill assertive boundaries from day 1. All adults have to have assertive boundaries at work at some point, it's part of adulting and growth to be able to. Bullies quickly retreat when met with calm, assertive boundaries.
Also, DH new workplace,like the vast majority now, should have cameras . DH should familiarise himself with where they are, do they have audio? And be smart about positioning when choosing to interact with this visitor,if needs be
.
Otherwise, just a brief, neutral "hello" and no interaction with this visitor would be a healthy choice.
You can't really reassure your DH,but you can research how to control your own anxiety. It sounds like there is a high level of anxiety for you both? You might need professional support with the lies anxiety tells you.
DH , as all adults do, has got to do the work himself to teach himself how to feel more secure and assertive, nobody else can do it for him. He's lucky he has an understanding,supportive spouse because many of us have to literally fight to survive serious ,life threatening physical , sexual and mental childhood abuse completely alone. If we can do that, adults can reclaim their own power .