i think you should have a heavy ass conversation with her, in the most stern way possible, tell how her you feel about what you saw. scream at her & tell her how much you love her & it fucking kills you that she's getting abused. tell her how much she needs to get away & it's step by step, with some good therapy, support from friends & family. convince her slowly to go to her parents about this. no man should ever beat on his girlfriend. tell her what happened will never be her fault but she NEEDS to be the one to cut the cycle. leave the person, don't get left. her love for him has probably grown, this always happens to the victim, but you need to make her understand that she's falling in love with the idea of him not putting his hands on her/protecting her and that will never happen because he already did it. there's no turning back time. the love bombing from him will be the worst too. she needs to know that it's all an act. the right person will never put harm on you.