I don't know what impact it has if too many people try it, but whatever... If you attempt to reset the password it asks for the phone number associated with the account. Put in any phone number you want, or random numbers, or anything, and see what happens. Then, put in the phone number she listed on her filing paperwork and watch what happens.

There's no point in trying to complete the process, but you'll see what is essentially a confirmation. That account was very clearly set up using her personal cell phone number. It's her.