From: Rabbi Moshe Sofer
To: Rabbi Nosson Shapira
The Chasam Sofer rules that the widow of a thoroughgoing apostate who never formally converted is permitted to marry a kohen. Surveying all major opinions, he concludes that under the mainstream view the mumar remains a full Jew whose marital relations carry no disqualifying effect, and even under the most extreme minority view no Torah or rabbinic disqualification applies to his widow — since Chazal would never enact a stringency that closes the door before ba'alei teshuvah.
Source: Shu"t Chasam Sofer EH I §93