Bought a Mac from a widow. Her husband died with the password.

Nope' not with out access to the husbands password and Apple account.

If the widow does not have the husbands password.

Then she needs to request access to her husbands account.

see > How to request access to a deceased family member’s Apple Account - Apple Support

If the widow can find the password or requests and gains access.

Then she need to properly erase and prepare the iMac for resale.

see > What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Then and only after that, can you set up the iMac as your own using your Apple ID.

No, there isn’t to be honest you have an unworkable computer because it will always be associated with the original owner. If it is appropriate, return it. A used computer MUST be setup per Apple’s directions in What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

This means NEVER buy a used Mac from:

Amazon

Ebay

Marketplace

Local PC shop

Friend or neighbor

because more often than not they have not been set up correctly. If you want to buy used either buy from Apple’s refurbished store or from a reputable reseller such as Macsales.com