Unfortunately for Humbert, Lolita's friends are really not nymphets. Her best friend, Mona Dahl, is a little too experienced for Humbert's liking.
She also doesn't betray any secrets about Lolita, so is not useful to Humbert as a source of insider information. After interrogating her, Humbert can only detect that she has a crush on him. He worries that Lolita is setting him up.