I'm sure you've considered this but have you really come to terms with the idea that he will be taking over all aspects of her life. Not just how she dresses, who she associates with, but also who she fucks. It is essentially a forgone conclusion that he will begin having sex with her, and more than likely she will no longer be interested (or permitted) to have sex with you. I'd wager that the point of no return has already occurred in the sense that her view of your has shifted and she will no long ever view you as an ideal sexual mate.
My personal experience on the topic is that even when I attempt to be commanding and dominant, my wife responds with a sarcastic remark or a "ha" via text. In bed she will suggest we "snuggle" instead. This has gotten more and more frequent and is pretty much the norm now. Her boyfriend is very jealous and has basically ordered her to never have sex with me. Pretty much every night after we go to bed and she rejects me, I end up jerking off next to her while she sleeps - thinking about her fucking other men. It has gotten to the point that I rarely am able to cum without imagining her being fucked by others.