The only way was to find the aufere malus spiritus ritual to slowly remove that thing from his body, but he needed two strong casters, in order to remove that monster. Quirinius knew that Severus and Filius were strong enough to do this ritual. And he would be able to trust them, because Dumbledore was the one, who brought him in this situation with his compulsions and Minerva was too loyal to this old fool. Promona wasn't overly strong, but she would able to find the ingredients in her greenhouse due to the fact that the ritual needed potions and all the potion ingedrients could be found in the greenhouses in Hogwarts.