I think you'll find if it was her son they'd have cracked him long ago, they're extremely professional at that sort of thing nowadays (without any longer having to use the 1970s bully-boy tactics).
I noticed that the man in the yellow slick tried to use her card at at atm later that night, but failed to get any money. So evidently even though he terrified and tortured her she didn't give him the correct number.
I hope that was on purpose and not because she was so terrified she couldn't remember, bless her heart. Damn it, people who torture old people and children should be locked up and have the key chucked away for good!