Is this that weird? If you completely lose your memory, you're bound to feel anchored to anyone you feel can give you a sense of self. He was with her on the train and was probably helping her through the whole process, plus she's probably genetically attracted to him in the first place. And they're probably still dating on Facebook. I guess the only thing that could ruin it was if he was a huge asshole.
My thoughts.
If they went their separate ways shortly after she got amnesia, and they fell in love again upon coincidentally bumping into each other in a cave in Africa years later or something, that would be a story. Otherwise, you know....