There are some issues not clear to me, you are saying girlfriend, by girlfriend do you mean a friend who is a girl? In usual sense a girlfriend term is used when there is a boyfriend girlfriend relationship, usually an exclusive relationship. I dont know how old are you and what intensity of relationship you had with her and having now, and might have in future too.
The other thing not cleared is how he is blackmailing, is it some record of sex scene or what? If you could explain that would be better.
Now your concern, I am not keen about the names, all I can say is an engagement is not marriage, it can end without any hassle. Even if it was a marriage a divorce was okay to proceed. There is no marriage, a guy broke his trust, there is no reason to have any future.
Give a suggestion to your friend to contact Police station, there are special cell for women and children, report on that. A lawyer can give all legal help with all the IPC codes applicable. She can start with section 420, and some more like postering to be someone else to engage in sex without marriage. If there are some video evidence, its better to get the guy a real shit under cyber act too.
Nothing is lost for her, she can get everything back, even a man for her, as you are there as her good boyfriend.If you are not married, stand beside her and accept her at this bad time.