From the posts above it looks like the inheritance went to her British husband only, so if he is not a "US person" then surely he should not be expected to file a 3520. Unless the will stated that the inheritance should go to husband and wife I don't see an obligation on the wife's part. Even if the husband shared the inheritance with his wife then that would be a gift from him to her and subject only to UK gift tax law but she says he moved into his own savings account. It just happened to pass through the joint checking account hence the need for FBAR.
If he is an NRA and they do not elect MFJ, then no reporting on his part. The problem is however is taken from the below 3520 instructions:
You are a U.S. person who, during the current tax year,
received either:
a. More than $100,000 from a nonresident alien individual or
a foreign estate (including foreign persons related to that
nonresident alien individual or foreign estate) that you treated as
gifts or bequests
Can it be argued that the OP benefited in the 'including foreign persons related to that
nonresident alien individual'
Also:
Related Person
A related person generally includes any person who is related to
you for purposes of sections 267 and 707(b). This includes, but
is not limited to:
• A member of your family—your brothers and sisters,
half-brothers and half-sisters, spouse, ancestors (parents,
grandparents, etc.), lineal descendants (children, grandchildren,
etc.), and the spouses of any of these persons; or
• A corporation in which you, directly or indirectly, own more
than 50% in value of the outstanding stock.
Just my reading, but for a few hundred dollars to file the 2020 return, certainty with a specialist expat firm would be my way forward...