My husband called a co-worker Tuesday night after 10:30 pm and talked until almost midnight about business. This person was also on vacation. This was not an emergency call and I feel, regardless of whether the co-worker kept the communication going, that my husband was out of bounds in making this call. Please advise.
It totally depends on the culture of your husband's workplace.
At some offices, this would be beyond the pale. At others, this wouldn't be particularly strange (especially, for example, at a lot of start-ups).
At my last job, many people would do some work from home at odd hours of the night (including me). I had one coworker who I knew often worked late at night, and sometimes he and I would find ourselves exchanging work-related emails at 11 p.m. and finally he'd suggest we jump on the phone to get something resolved faster. (I never suggested it, because as a manager, I didn't want to make anyone feel pressured to give up their nights that way. But if an employee initiated it, it was fine with me.) But that was just our culture, and it was the two of us in particular -- there were other employees who I never heard from after 6 p.m., and that was just fine too.
As for the vacation element, again it depends on culture. Sometimes I go on vacation and make it clear that this is a "don't bother me unless someone dies" type of vacation. Other times, I want to get away but know that my workload at that point means that the only way I can do it is if I remain available by phone -- and in those cases, I'm willing to make the trade-off of getting to fly off somewhere fun in exchange for remaining available for phone calls.
And there are certainly people who enjoy their work so much that they want to stay in touch while they're away. I've been that person myself, and I've worked at places like that; they do exist!
So it really depends on the workplace culture, and the preferences of specific people involved.
Now, despite all that, if your husband had no basis for knowing that this coworker would be fine with a 10:30 p.m. work call, then yes, it was inappropriate. And even if that's not the case, if your husband has any authority over this coworker, he should be sensitive to the fact that the guy might not feel comfortable saying, "Hey, it's getting late and I'm on vacation. We need to wrap this up." Or even, "Hey, this is my vacation. Stop calling me! We'll talk when I'm back."
In general, people in positions of authority should bend over backwards to be respectful of people's off time, to encourage people to take "real" vacations, and to make it clear that working odd hours is truly 100% optional. So your husband's role in relation to the coworker is relevant.
But some people really do like working at odd hours. Some people do not. The question is where this guy stands.