This happened to my daughter. That man had her behaving totally out of character within a fortnight planning to throw her degree away and move in with him.
He told her that her friends were idiots and her family were controlling and didn't want her to have a boyfriend.
He kept tabs on her constantly, locked her in his flat, took control of her phone and kept hold of her car keys. He worked constantly to drive a wedge between us and prevent her from seeing us and talking to us.
Within two months he was planning a wedding with her and she was hoping that we would say no because she couldn't find a way to do it herself. My daughter believed she didn't have the right to choose who she married and when. That is not how I we taught her to think and will never make sense to me.
He persuaded her that she had to do exactly what he said and dump her old life to demonstrate her loyalty to him.
His behaviour escalated to violence and serious sexual assault and she kept defending and excusing his actions.
I worked my socks off to keep the lines of communication open and make sure she know we were there for her whatever happened.
I was with her on one occasion at the right time to support her when she realised what he was doing to her. With a lot of support she ended the relationship at three months.
Months later, she is now recovering with support and counselling. He has found another vulnerable young woman, done the same to her and married her against her family's wishes 
Don't judge her or blame your friend; support her. Find ways to let her know you are there for her if she ever needs it. Ask questions instead of expressing opinions. Don't make her choose between you and him or do anything that may make her feel disloyal to him. Maybe one day she will feel able to walk away and put her life back together the same way my daughter has.
Don't question why she let's him do this to her. There isn't a logical answer. Strong women in high powered jobs can spend years in relationships like this. There is something that makes them hang on to the few little bits of good in the hope that, if they make more effort/are better people, the abusive partner will be nicer to them. It just never happens.