Her reaction to her husbands death will be influenced by all kind of things. They may have lost intimacy and been living like friends, he may have been really difficult to live with, she may have been desperately unhappy, he may have know he was dying for some time and wanted to help her move on, she may have grieved his loss long before he died. We don’t know what goes on in a marriage so to judge her for her reaction to his death is grossly unfair.
She may be lonely, wanting male company or she may be trying to lure your husband away - again no one can fully know. It doesn’t sound like your husband is hiding anything from you in terms of her contact with him or that he’s interested in her, so he’s not heading into affair territory by the sounds of it.
What do you want him to tell her about stopping contact with her? That his wife think she’s over reliant on him and might be trying to seduce him? Which sounds closer to the truth? I don’t think I’d be prepared to lie tbh so I don’t agree that he should say whatever you tell him to say about it - you don’t want her contacting him, be honest with him and her about why. If your friends think that makes you a nutcase, so be it.
She’s recently widowed and trying to find a way through, grief affects people very differently - there’s no rhyme or reason to how we get through that, not knowing the content of the contact it’s impossible to say that there’s anything dodgy or skanky(!) about her behaviour, but your attitude to your friend leaves something to be desired.