That sounds like a condition laid down by O2 rather than the Halifax, I don't think it would make any difference what bank your neice held her account with. Has she had any credit before? If so, is she up to date with payments?
I recently set up a credit account with British Gas and have had to pay a ddeposit of £250 which I get back after a year, as long as I am up to date with my gas/electric bills.