Have you been sexual with this friend or is it just a close friendship where he wants it to go further than you are willing or able?
My wife once told me that if I cheated on her, that I should keep it a secret. She sees me having to hide that as punishment for me. So you will have to decide what to do based on what you think your husband wants to know.
Do you think your husband would ever accept polyamory?
When dealing with pain from losing someone, I find that time heals wounds (or at least dulls the pain). Also do many activities to occupy your time.
I wish you well.