I think she should have a heart-to-heart with him. Take him on an emotional rollercoaster.
If she loves him, then maybe it is killing her inside watching him grow older every day, knowing that someday he will die and leave her alone again. Let her explain this to him while tears are pouring down her face, and see how he reacts when she asks him just to think, to
consider, staying with her forever and never leaving her alone to face the battles of life without him. Give him some time to mull it over, emphasize her sad looks whenever something happens to him that only emphasizes his human frailty more. Most of all, be patient. After a few weeks or more of game time, if he hasn't responded, have her leave him in a drawn out break-up and become somewhat morose and withdrawn. Have her then reconsider the pact she made as she grows somewhat bitter about humans, and join the dark side where she can then discover some dark part of her actually enjoys dominating the masses. That is just some excellent roleplaying waiting to happen when they meet again. Will her bitterness rule her forevermore as she spirals downward, or will he somehow bring her back from the shadows? Or will he just stake her and move on as if she meant nothing to him?
Of course, the believability of this depends completely on the personality of your vampire, which we don't know a lot about. Just remember, though, making players cry is fun. I fully endorse it!