You can try to talk to her, and convey the importance of an education, but until she is at least self-motivated, none of this may do any good. It sounds like she needs to make her own decision to be in school for school's sake, and not for a social life.
The best advice would be to advise her to keep focused on schoolwork and to hold off dating for another time. It makes more sense and it's more feasible to look for a serious partner after school because a career takes people in crazy directions and they might not even end up in the same place.
Well, if she keeps getting D's her gpa will go way down and she might be on academic warning. Tell her to just take 4 courses in college to remain full time instead of 5 or 6.
Take some easy courses in her major to lighten her burden and raise her gpa back up. It might feel good to see her name on the Dean's list if they have one.
step back and show her love. It may not change things for the better in current circumstances but it gives her the ammunition to learn from it no matter how things end up.
Did you tell her.. top marks or you have too pay rent?