Still wouldn't be my problem, her taxes are her responsibility. I am not a CPA and did not put my signature on her taxes. Any advice she "allegedly" got from me is meaningless.
Just warning of a potential issue. Take it for what you will. This is a common tax shelter game and usually ends with audits on both ends.
Audits are a painful experience in both time and opening the door for future audits. Keep in mind any records you don't have will screw you. Also although people think 7 years is the limit, they can keep going back another 7 years if any misreporting or fraud is found.
I would just be clear to the nanny that you will be reporting her wages via 1099 and she will need to file taxes on them.
What happens to instigate this a lot of the time is the nanny gets a surprise 1099 when they were assuming it was a 'cash / under the table job' and the hirer lead them to believe it. They negotiate a low rate and then the sitter gets screwed.