Hmmm — something doesn’t sound right. Typically, conducting research requires some stringent guidelines, even at the exploratory level. At the very least if this is a serious inquiry about criminology someone should be guiding her. If this is just her own thing she seems to be doing just what she pleases without any regard for protocol, your feelings, or any sort of integrity in researching people.
If she isn’t in a program and engaged with an institution to guide her in doing this research, then I’m not sure exactly what she’s doing. If this is just an interest and she is on her own and using a flirting style to get a response, that doesn’t fit any type of research in psychology, nor does a subject offering to help pay for college and her being excited or interested by that.
This sounds more like her own project, with no guidelines, and letting you read the emails, if it were real research, would be at best an ethical violation. You explaining to her how uncomfortable and her brushing it off saying that no one is going to stop her, pretty much gives you the answer. You’ve told her this isn’t okay and her response lets you know she doesn’t care. Whatever it is she thinks she is doing doesn’t have the relationship or your feelings at the core of it.
If she doesn’t honor what seems like a very honest and real request from you at the 6-month point, why would you continue watching her do this, hurting your, and not caring about anything except her own agenda?