I think that person is Chanel No. 5
1. She replaced the poisoned apple cider with a new one.
2. She did some manipulations on the cryotherapy machine.
3. She intentionally left her phone on the table at the lingerie boutique so she can't answer Chanel's call and started arguing with the saleslady to draw the other Chanels' attention to her thus leaving Chanel alone in the pool with Dean Munsch.
I still can't figure her connection to Dean Munsch or her motives for saving her.