I screamed at him to stop and Cindy started freaking out. Finally, a couple of guys made him stop. Cindy ended up with a black eye and a swollen lip. When we left the party they were acting like nothing happened. It was weird.
Physical safety is the first priority.
Hitting and beating usually get worse as time goes by. Ignoring a beating is dangerous. Explain this to your friend. Listen calmly and take the concern seriously. Reduce her vulnerability.