Optional: A different timeline.
What if their employer already hires the heroes just after the departure of her old friend? The idea would be that they maybe even find the old merchant in time who is still willing to do the ritual. Whatever the heroes will say, the old merchant will not let himself persuaded to not do the ritual. He will instead lie and say he does not know anything, and try to shake off the heroes.
When the old merchant thinks the heroes do not see him he will go to meet the priest. The heroes could then secretly follow. The old man goes then to the farm but never comes out. Instead after a couple of days a large wooden box is brought out, big enough to carry human (the old merchant inside with sleeping because they gave him a long lasting sleeping posion). It gets loaded onto a carriage and one of the young guys loading the box wears the old merchants clothes. From here there are many possibilities how to proceed.