The reason the narrator does not disclose to them that he knows their secret.
Note: This question is to be answered on the basis of your own understanding, experience and thoughts. It is strongly recommended that you prepare the solution on your own. However, some pointers for discussion have been provided for your reference.
Nicola initially is not comfortable with the narrator driving them to Poleta.
After some thinking they agree.
The moment they reach their destination they jump out of the car and do not ask the narrator to come with them. On the other hand, they suggest him to spend time at a café in the village.
The narrator understands their reluctance to share their purpose of visit with him and, therefore, does not disclose to them that he knows their secret.