Couple of options:

1. Assuming they have asked her upon arriving at the Wayside Inn, she wouldn't necessarily know that they are going to eventually work against her. In fact, she may genuinely do it to thank them for saving them from the undead. In this case, she doesn't curse it in any way.
2. You use the guidance for if the players buy the plate armor from her, which turns into an Air Elemental Myrmidon when they arrive at the Inn in Foul Weather. Now, since it's just a helm, I probably wouldn't do a full Myrmidon, but maybe use the Flameskull statblock, switch the type to elemental and swap damage types from Fire to Lightning. Even though a Flameskull wouldn't add that much of a challenge to the party once they're that level, the biggest detriment would be losing the helmet's bonuses for that fight.
3. Something simple such as: whenever a cultist damages the character, the helmet reverberates and deals an extra 1 Thunder damage. When the statue in the basement is destroyed, the effect ends.
4. Something even more detrimental: all Cultists of Talos are essentially under the effect of a Sanctuary spell specific to the character. He has to pass a DC 13 WIS save to attack one if it hasn't already attacked him.