Yes, in the way he left (implying he died in battle), what she is doing isn't borderline, is an obsession, there is a psychological term which I don't remember. But definitely what she did, is not what a mentally sane person would do.
remember that she is influenced by the bond they made. Yes in human terms it seems like a obsession caused by a psychological reason, plush him going missing. but remember that it still is caused by the magic bond (don't remember what it is called) she made with him before her death, or at least it is implied that it is the sort of magic bond that effects a person, and even if it doesn't, it clearly means something in the elves culture at least (my guess is that it is a type of magic that an elf couple will make to ensure that they will stay together forever or whatever.) also that magic bond was the thing that told her that he is still alive.
yes in the modern world her actions can be considered as stalking, but remember that this does not take place in the modern world. (also even if it was in the modern world, legally it's not stalking because it does not effect him negatively in any sort of way, and it is not repetitive either, which also makes it 'not stalking' in a psychological way too, or at least not yet) the setting is also medieval so, there is no way this would be considered stalking in that case.
so it's still not stalking, but let's say that he didn't want her to follow him, but she still did follow him, it would be because of the bond and not because she wants to follow him. (which i doubt she would follow him if he told her that he doesn't like her, or expressed any sort of disgust towards her. so it wouldn't be called an obsession on a psychological level, unless i am wrong and she would still follow him.)
edit: what i mean by it doesn't effect him negatively, i mean that he doesn't dislike her following him.