I would recommend that you take her to some colleges that offer theater majors. Help her set up some interviews with college theater faculty and theater majors so that she can hear from the "horse's mouth" what it takes to be successful in college theater and professional theater. She would also benefit from talking to professionals currently working in the kind of musical theater she aspires to enter. Encourage her and help her to contact theater professionals to ask them if they could spare a half-hour to give her some advice regarding her musical theater aspirations.
If she has a passion that is fueling her musical theater aspirations, it's wise to encourage that passion. I know that doing that while you have legitimate worries about her future without a college diploma is hard to do. Maybe if you step back from what appears to be a standoff and try to help her connect with "real life" theater professionals, she will open up more to you. Your behaving in this manner might convey to her that you are really trying your best to advocate for her dreams rather than just getting her to go to college.