Piggybacking can’t hurt your credit, unless…
Piggybacking is pretty safe to your credit score because you are still in charge of the account. As long as you keep making timely payments – nothing can go wrong with your credit score.
If you only plan to make her an authorized user but don’t let her actually use the card (FICO doesn’t care about her actually using the card) then no worries.
However, if you actually plan on giving her the card so she can use it, make sure to ask for a CLI so that your CL is 10-15 times your total combined monthly spending. Otherwise your utilization will go above 30% (even if you PIF) and that can certainly hurt your credit score.
See this post about the required CL.