May I recommend you all look up your citizens arrest laws, sexual assault's carry harsh penalties.
Here in Perth for example (oh shit quick I am not a lawyer disclaimer) If I was with an acquaintance who was sexually assaulted I could respond as follows and be completely legal (assuming I have video or human witness to my actions that will corroborate the events.
1. Assault occurs, I see it
2. I say in a loud clear voice to the assailant "You are being placed under arrest for sexual assault, this is a lawful citizen's arrest and I will hand you over to the first available police officer".
3a. Assailant comes quietly to police officers/station and gets charged. [end]
3b. Assailant laughs you off and walks away.
4. You announce to assailant that "Failure to comply with verbal requests is resisting arrest and I will use force to prevent your flight"
5a. They get physical
5b. they run away
6a. You restrain them on the ground and have someone call the police to your location.
6b. You chase them and restrain them on the ground and have someone call the police to your location.
(this is super simplified and there's many situations that you need to know more specifics, but it is legal for you to arrest criminals that commit serious offences and escort them to police stations)