rashi on 44 says achan was bad to people and to hashem
he prohibited her to her husband and embarrassed the family
which is bad to people and which one is bad to hashem is an affair that prohibits a women to her husband something that cant be atoned for without the husbands mechila!
Unquestionably, to atone for the pain that he caused the husband the adulterer needs the husband's Mechilah. However, this does not make it into a Mitzvah "Bein Adam l'Chaveiro," for which one cannot receive atonement without Mechilah. Rather, the Aveirah of Giluy Arayos can be atoned for, but in addition to that Aveirah the sinner must also seek atonement for the Aveirah of causing pain to the husband. To achieve atonement for this latter sin, he must ask the husband for Mechilah.