I would have a sit down with her and have a heart to heart, state clearly that the way he's treating her is not normal and that he is abusing her. Let her react whichever way she wants but end the conversation by telling her that you will always be there to support her if she needs it. It is very important that she doesn't wake up one day and feel trapped with this man with no support to get out.
After this, if she dismisses your concerns let it rest, don't keep bringing it up but also don't encourage the relationship. It's important not to show your resentment against him too much/ refuse to socialise with him, as he can then make himself out as a martyr to her and start telling her that her friends are toxic, don't respect him, etc.
Don't let her end up closed off from the rest of the world with him being the only thing happening in her life, keep making dates to see her, keep inviting him to the occasional meet up, keep messaging and calling to see how she's doing. It's heart-breaking listening to someone you care about describe how they are being abused whilst also refusing to acknowledge the situation, but don't get angry and cut her off however hard it is to listen to.
Just be there to listen, often someone saying the things that are happening to them out loud to someone else is the only time they can actually hear how bad their situation sounds.
If you have a loving partner yourself, subtly mention all the functional, normal things he/she does for you, helping around the house, asking you what movie you want to go see, etc.
It all boils down to: be there for her and don't let her become alienated. She needs to continue being surrounded by normal people who she has functional relationships with if she's ever going to eventually see that what is happening with him is not right.
I sympathise, my friend had an extremely traumatic childhood and still lives with the family members that abused her, as you can imagine it has led to a very unhealthy relationship with a man who does not treat her respectfully.
All you can do as a friend is be there for her.