One of my friends has BPD/OCD and recently broke up with his fiance. I see signs and speech that he may start to stalk her. I am very worried. Is stalking part of the behavioral spectrum for this pathology? What steps need to be taken to prevent this?

This being a topic close to heart. Obsession comes in many forms and it is in our minds that we manifest it taking form into anything we want. It's not within the behavorial spectrum of someone with OCD to stalk a human being. A human being not being a possession. We obsess only of material things with no value or mementos of which we are afraid to let go off which bring comfort/ease to our minds, such as the case with hoarding.

A definition of obsessions within the spectrum are as follows:
These are recurrent, persistent and unwanted thoughts that cause the person much anxiety/distress.
Besides coming under the form of thoughts, they can also be images or impulses that will repeat themselves and are experienced to be unwanted, intrusive and inappropriate.
The person feels overwhelmed by the obsessions as they seem to intrude in every aspect of their thinking and are very disturbing thus triggering much anxiety and discomfort.
They will try hard to suppress the thought/images/impulses and/or try to "neutralize" them by other thoughts or specific actions.
They do realize the thought to be a product of their own mind, although with children it can be they sense it to be "a voice" that is telling them to do bad things.
The Obsessions and what they stand for are often in harsh contrast with the person's own convictions and beliefs which makes their impact only greater.
Obsessions are accompanied by uncomfortable feelings or wanting things to be done "Just Right".

However, I may be wrong. Hope that your friend seeks help.