Also:
--Durkula doesn't know she's there, and has planned for a party without a medium to high level cleric (the escaped vampires know that *a* cleric helped drive them off, but no specifics as to power level, and Durkula would have no real reason to know that Durkon knew her);
--Durkula can't see anyone but Roy per how Sending works, and will therefore not get a clue that way;
--Durkula is in a sealed room behind stone walls and a heavy door, so even if Hilgya reacts angrily that shouldn't give it away;
--Durkula won't even know who Hilgya is or that she slept with Durkon unless he happened to look at the specific memories involving her, which seems unlikely (but not impossible).