I have a FirstUSA BA (formerly CHASE BA-Visa) card since a couple of years. The miles are posted to BA before I even get the paper statement.
The statement shows how many miles earned in that period; the miles are always correct posted.
She should have gotten statements from BA - even when she was not flying!
As somebody already suggested, the miles may be sitting at BA in another "dead" account.
I would call BA again and let BA search for the account, and may be send BA the old statements. I believe the miles are not lost.
BTW, FirstUSA's agents/supervisors are not very friendly. I had a "discussion" with them about the amount of interest rates they where charging. They where sending out checks with 3.5%, and I deposited them to my 5.75% bank account. However, FirstUSA charged more, I still made a small profit.