You start with the spouse's name. Then the kids' names. Then start going through various combinations of the kid's initials. Then start trying nicknames.
If the person wasn't taking password security seriously, it's possible to guess.
The other option is knowing enough about the person to trick the Yahoo! password utility to provide you a new one.
It’s damn near impossible to do all those combinations and get it right.
He most likely got through on the Yahoo question she had set up for if/when she ever forgot her password. The questions are pretty basic like, “What is your father’s middle name.” Then Yahoo lets you reset your password.