They could have grown up together, had shared classes, graduated the same year. Then she went off to the big city for college, but (being a bit naive) got roped into the cult, while he stayed home or went in the service and came home to be a cop.
The cult could have started out as a main stream church but hired a charismatic minister who led them further and further away from the normal tenets of that denomination until they became a cult. Two ways this could happen. One, the national church evicts them from the organization or, two, the pastor talks the congregation into rejecting the organization and withdrawing from them.
If you go with the cult being local, she could have escaped from them but, being poisoned, collapsed and he found her while on patrol.