Obviously the activity of Fr. Duverger, if true, is sinful, but apart from that the big issue is that he uses his position as confessor and spiritual director as leverage to persuade his victims to engage in the behavior. So a penitent could be sucked in to a point until she realizes what's going on and that a line has been crossed, through a certain gradualism. At first the reaction is confusion. "Well, that's extremely personal and involves some graphic descriptions, but perhaps it's necessary in order to understand what's going on with my spiritual life." It starts with stuff that's probably borderline or ambiguous and then slides over a line before the victim is aware of what's happened.
But that's not the real story here. People are prone to sin and to falling into sin, even priests. I'm sure that the devil tempts priests more than average lay people due to the harm he could cause through their fall.
Let's not lose the real story, the coverup and the reassignment of Fr. Duverger where he had contact with parishioners and children despite having become aware of prior similar allegations. Fr. Wegner and Vogel both admit that Fr. D has a history, and Vogel admitted it would seem "bizarre" that he's been stationed somewhere he could have close contact with the faithful (and even children) at a school. Vogel should be fired, and Fr. Wegner sent to a monastery and removed from any position of authority in the SSPX.
For as credible as this Jassy SEEMS to be, one could perhaps wonder whether she could have fabricated or exaggerated or distorted some things. Taken alone, it should be with a grain of salt ... although it should be investigated. But when there are multiple similar accusations, the amount of smoke should be understood as indicating the presence of fire.