You said, "She is not my student.."
So there is no moral dilemma. Outside America, it is okay to have considerably younger women, as long as they are of legal age.
So look upon this as a moral obligation. A duty. You owe it to that young woman to service her properly!

It doesn't matter as long as she's a student in the same university (and I've seen that in legal terms, because you can't be the gym teacher and then just go around f[edited]ing the English majors and then it's OK.). I can tell you from experience that any student I met in a Chinese university thought of me as their teacher whether I was or not. Some students I taught one lesson to, or just was introduced to by my students one time while passing. After that, I was their teacher. You have the same respect, power and influence whether they are in your class or the class next to you.

Nice try though.