It very well could be a symptom but be very careful it could be symptomatic of a lot of things. I know that my wife (BPD) engaged in this sort of thinking when she was in her late teens, as she showed me her diaries and it was just a real bad obsession with someone. So my guess is that a very strong obsession could be part of BPD but it also could be part of other things. Read the book Stop Walking on Eggshells or some other reference material and see if this person is doing other behaviors if she/he is doing 4 or 5 sorts of things that have no other explanation then maybe you are onto somethign.