The landlady did not harm Mulholland and Temper in the traditional sense. However, there are strong indications that she did something sinister to them:
* She serves them tea with a strange, almond-like taste. This suggests potential poisoning.
* She seems overly enthusiastic about them staying and insists on keeping their coats. This could be a way to prevent them from leaving.
* There are no other guests in the seemingly full house. This implies that the "guests" are not alive or are kept there against their will.
* The landlady's taxidermy collection is uncanny. She seems to be obsessed with preserving the appearance of her former guests, perhaps literally.
It's left open to interpretation what the landlady did, but it is strongly implied that she murdered Mulholland and Temper and preserved their bodies. This is a classic example of suspenseful storytelling by Dahl, where the reader is left to imagine the horrifying reality.