S libby entered the library, her friend saw her and raised her finger perpendicular to her lips to indicate to libby that she needed to remain quiet. which element of kinesics is this

I believe the answer is substituting function.

Substituting function refers to the replacement of a certain action with another that served the same function.

From the situation above, placing your finger on the lips is a generally accepted substitution gesture that indicates a request for other person to stop talking/lower their voice.