Answer: Substituting
Step-by-step explanation:
Substituting is the mechanism through which someone serves as the substitute of other person or thing .Acting on behalf of other's place for completing the function instead is the duty performed by nay substitute.
According to he question,Libby's friend is acting as the substitute in library space to indicate and impart the information about keeping quiet in library before Libby can speak.Libby's friend is serving the function of library guidelines.