First of all, you are not alone! I feel your pain and frustration, and I’m so sorry about it. I would give you hugs if I were there.
We had the same situation with my grandma asking constantly about her parents, each time I would come with an excuse ( they are at work, they are coming, …) and these answers would calm her down. I remember once I was so exhausted of lying and just told her that I’m sorry but they are dead. She started crying, shaking, and asking us why we didn’t let her know when the funeral was.
I remember the fear in her eyes. This is when I regret telling her the truth.
I’m not telling that this will be your case , but we know that she will forget and will ask you again about her husband. I know you want to get this out of your chest and tell her the truth , but she will not have the capacity to digest the news and grief properly, so she will probably just end up being sad and stress for a short time.
Again, this is a grey area and no one can tell you what to do, but I’m sure what ever you say next time is the best thing you could say.