How do we know she didn't?

There are reports that the actors who portrayed Kali and her gang have been spotted on set filming season 4.

@Fisher Fan we will never know

@Averhoes I didn't see that

@Fisher Fan we don’t know a hundred 100% but when hopper asks where she got her clothes she changed the subject and didn’t tell him about Kali. She could have told mike or max or else in the party, but as far as we know she hasn’t told anyone. Plus it wasn’t mentioned in season three.

I do see that because since I heard! Kai and her gang be spotted on set, I feel like they are involved getting Eleven back to the gang.

@Zimmydude true, she didn't tell him in that moment. But with touchy subjects, it's often natural to wait a while before saying anything or disclosing sensitive information. I wouldn't be surprised if Hopper had casually brought it up one day when she was more comfortable with having a conversation about it.

But as you said, we don't and won't know for sure unless it's specifically mentioned in Season 4.