As much as the OP doesn't want to hear this, he can't "do" something.
This is adult woman with full agency, and gets to make her own choices.
Personally, I think the term "friend zone" was mostly coined by men that are/were bitter that the object of their pursuit had the audacity to take things at her own pace.
Then, females took it up with each other as a way to "let him down gently"
Essentially, it's an excuse for both genders to avoid saying or hearing "I'm not interested in you." In a word, it's cowardly.
I wonder how the OP would feel if someone was asking how they could "make him" interested in her again?
Is there something wrong with remaining friendly, cordial with each other until one of the parties figures out how they feel?
Have some patience for gods sake.