I use BTconnect, but the set=up is the same I think.
There is a webmail facility
I assume you have BTopenworld as your homepage - if not go there
There should be a login link - Enter your main user name and password
Look for a link to webmail (it's ususally on the top row) or search for it
a new window should open
Enter your wife's email address and the password the BT supplied for the 2nd account
You should be able to see who the email is from and the size.
You can try and look at it on-line, or you can delete it on-line if you don't recognise the sender.