Well it really depends on how the party views him. If he seems like a victim of the cults machinations the he most likely is an ally. But, if he comes off as either too vengeful or too charismatic then some flags are gonna be raised, especially if they find out that he is a part of the cult. So I would play him off as a noble who has been targeted because his goals come into conflict with the cult. And since the party seems capable then he would be interested in enlisting them against the cult, and is willing to pay a handsome sum of money when they are defeated. In reality though the missions that he sends them (the party) on further his and the cults end goal.