In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, the Fatwa Center at Islam Q and A, states:
It is permissible for your husband’s friend to marry you after your iddah ends, and this is not regarded as a betrayal of his friendship.
Rather, this is something that many people do as a way of honoring the friend who has died, especially if they left children behind, so they want to marry in order to take care of the woman and help raise her children.
If the husband intends to honor his friend, help raise his children and help his wife to maintain her chastity after he is gone, he will be rewarded for that.
It is no secret that what is forbidden is for the woman to establish a relationship with a man who is not her mahram, whether he is a friend of her husband or otherwise. This is what is regarded as a betrayal and infidelity.
If this man wants to marry you, and he is religiously committed and of good character, then he should go to your wali (guardian), and you should pray istikharah, asking Allah to guide you to the right decision. And you should cut off your ties with him from now, until he becomes your husband.
Almighty Allah knows best.