You won't get anything across to him until he's ready and willing to listen - Usually not until she has split him so badly or so often that he can no longer deny/rationalise/avoid recognising that there is something very seriously wrong here.
In the meantime, you could gather some of the info/links on BPD here together so you can give them to him when that time comes. The usual caveats on not being that guy who sticks his oar-in without welcome apply, so take care of your own position and just leave him to draw his own concllusions (if any at all) once you have.
And yes, some more info on how her potential BPD is manifesting itself to him would help.