Do you know where they were living in America?
Have you searched for his death there?
You don’t mention any dates ...
Do you know when she returned to England?
Do you know when she married?
Do you know when the child was born?
A time line might help clarify.
There are any number of scenarios that could have occurred .... she visited England, her husband died in America while she was there, she stayed, years later met someone, they married.
I presume you suspect there is more to the story than that? (Eg .... she left him, ran off to England, started a new relationship, waited the seven years claiming she had not heard from her husband, presumed he was dead, then married the second chap). First husband may or may not have died.
I don’t think that you can assume that because she was pregnant prior to her second marriage, that she was not informed of the death of the first husband.
To start with, I would be looking for a death in America. If that fails, check England. Family stories sometimes stretch the truth a bit. You need to track down some documentation if possible.
Do you have her marriage certificate? Does she say she is a widow?
It might depend on dates as to whether or not she would need to prove his death before remarriage. I don’t know that any proof would be required - they would probably just take her word for it. That is my assumption - I am sure that someone will be able to give you the facts once we have some dates.