Is it permissible to lie to a friend to keep her away from a non-Muslim?

There is no blame upon you for what you did; rather, you will be rewarded for it, Allah willing. Lying is permissible to achieve a commendable purpose that cannot be achieved otherwise, and it may even be obligatory if thetaining the objective is obligatory. Dissuading your friend from the Christian is a commendable goal and a religious obligation. If you could not achieve that except through lying, then it is permissible for you.