I guess all you can do is be there to support her when she feels down or high. Also, as you're her friend you'll notice when her moods swing, and when/if they get to a dangerous stage, you'll be able to get her help. She'll probably not know when she's high or low, until she's returned to 'normal' and look back on it.
As she's been diagnosed she must be seeing a psychiatrist, so make sure she continues with the appointments, or goes on to a psychologist. As for meds, the norm is lithium but it doesn't agree with everyone and she may have to try different types, but then again she might not need it, I think these days doctors hand out medication much to easily, instead of spending time to find out the best way to help each individual patient.
Also, I guess the single most helpful thing you can do for your friend is treat her the same. Don't treat her any differently to how you did before she got her diagnosis. Let her know it's not the end of the world and that she can get through it - you both will - and that you'll be there for her no matter what. Although there is no cure, it's fairly easily managed and she may not get a major manic or depressive episode for so long, it seems like she's 'normal' (hate that word!). There are quite a few people with BPD who don't realise they have it - we all get up days and down days, but it's when the episodes are extreme that it's something to worry about.
In addition, tell you friend there are many well known people with BPD, such as Stephen Fry, Robbie Williams, Tony Slattery, van Gogh, many others. They're usually the creative kind actually! Stephen Fry made a documentary about it, called 'The Secret Life of a Manic Depressive' which is worth a watch if it's repeated.
Good Luck to you and your friend, if I can help in any way, let me know x