Should he write in the ritual book that he was previously a Christian? Or should he write that he was a Taoist? If he was a Taoist, is it okay to just write that?
A general rule: if someone was raised in an Abrahamic religion like Christianity or Islam or he ever converted to it even for a day, he should always choose that, no matter if that person became an atheist or whatever else later, because that person still has his mind programmed from that enemy religion and must renounce it.
Taoism is a neutral religion (I would even say that it is on our side, because it's founded on ancient esoteric knowledge like we are) so there's no need to have a separate category or renounce it.
So in short:
In the dedication page you either choose "Ex-Christian", "Ex-Muslim" or "None". Since your friend was a Christian at one time, he should choose "Ex-Christian".