You have to play these things by ear. We cant answer whether he is unreasonable or on in a situation like this. Each person will have a different perspective depending on their relationships
When you say 'they want to know how long he will be off' to you mean his manager does or HR want to know? Two different things, a manager is entitled to ask, he has staffing and departments to run.
If my job were precarious, and I were your DH I would try and find some middle ground. Does McMillan give any clue - as in its a matter of hours, a day, or a week? If it's the latter then I probably would have gone to work, and taken the Thursday off, assuming DM may pass over the weekend.
There will be many priorities at play here, job security, finances, and so forth. And the 'what ifs' - what if she doesn’t pass away as quickly as anticipated and is still with you in a week? What about bereavement leave? How long can he reasonably expect to take off without facing a disciplinary? Is a sick certificate feasible? Can he offer to use some of his holiday?
I really hope you can find a solution to this, it's another layer of stress he really doesnt need.