It is briefly mentioned in the story since I wanted to show that sometimes we keep things buried in, like Alonzo told her at the end “you are not obligated to tell me anything”. 9 years ago she killed her best friend, and 7 years ago she started becoming Clandestine. And he said he knew her because he met her in the club that day I think it was in the prologue, not because they had had any meeting beforehand. Hope this explains some of your confusion !