I am sorry for your loss.
It is hard to give advise because I don't know their side of the issue. It sounds like they are both being disrespectful and controlling.
I think you should decide what you want. Do you want to be with your husband? Ignoring this current women, could you be ok if he did date other people?
If going poly (with you being poly or mono) can work, then I think you will have to be more assertive. Say what you are comfortable with and what you are not. Read some poly books with your husband because that will help show guidelines for polyamory that tend to work. Tell your husband to cool it with the friend until you possible feel more comfortable with the situation. And insist they both treat you with respect.