Bella knows everything. She knows who Nika is. She knows who Summer is and where she is, and much more. The scene in the restaurant has several versions so far.
Vella lied, she did Nika's hair not for him, but for the girl who was once "in love" with him.
The meeting with Elsa in the restaurant was planned by Bella.
Version 1: Bella knew that Nika and Elsa had known each other since childhood and brought Nika to the restaurant to check Elsa's reaction. Would she recognize him.
Version 2: Bella and Elsa work together and Elsa came to the restaurant to check Bella's work and make sure that Nika looked like herself at the age of 13.