Wifi Risks after sketchy guy connected

Im tech-savvy enough, but not too well versed. My mom's friend and her sketchy boyfriend visited and he connected to our wifi. He seemed like he at least kinda tech-savvy because he was talking about bitcoin. Anyways today my mom's friends mother called and said he stole thousands of dollars from her. I made my mom change the router password and call the company, and to call her banking company too just in case. Im wondering how much damage this guy could do. And what I could do to minimize the damage.