Neighbor Touching Himself While Looking Through Window Into House

My friend (adult female) recently caught her neighbor (adult male) looking out his window into their house, staring at her and masturbating. Both houses are rental houses very close together. This is in Seattle. What should we do?

Edit: to add clarification, my friend noticed him staring at her and touching himself while she was eating at the dining room table. She’s not going out of her way to watch him. Additionally, another roommate in the house has noticed him potentially masturbating while watching her, but he was more hidden so it’s unclear. After the most recent incident, he came over and attempted to talk with my friend and her roommates. They quickly shut it down.