I have BPD and can relate to this some. For me it's because when I'm around other people I can read them a lot better and adapt to them a lot more, and "feed off the vibes" I guess is one way of putting it.

People with BPD often have anxious or anxious-avoidant attachment styles. I have anxious-avoidant, so there's the "want" of intimacy and the fear of it too. I think in person the want takes over and when I'm alone the fear takes over. Possibly. Just speaking for myself as something your friend might or might not relate to.