My mother used to beat me often and severely until I was approximately 12. My father pretended to play the"Good Guy" by speaking up and disagreeing. However, on the other hand, not only was he complicit, he was actually the reason I got beaten.
When my father fears a bad reaction or feels he may "get in trouble," he lies. As a kid, he'd break something and blame it on me when mom asked--a beating (with my father suddenly absent) would soon follow. This happened a number of times to the point that my mom eventually told me (after beating me) that she knew he was lying, she just wanted to see his reaction. He didn't protect me, he actively put me in danger. Willingly.