In this Q&A episode, Tucker Max, Nils Parker, and Dr. Geoff Miller break apart this question, the details he gave, and the assumptions he is making. They explain how if you see a guy who is to be subservient to or "pussy whipped" by his girlfriend or wife, you are only looking at a small part of their relationship and making assumptions based just on that. It's unlikely that you know their entire relationship history and all the dynamics going on. 
Sometimes, a woman is just awful or the man has given up too much control and dominance in a relationship (No More Mr. Nice guy is a great analysis of this problem). But ask yourself, "Why is this happening in the first place and why is he still with her?" Staying in that relationship is a choice that as a man you have to take responsibility for.