What to do for a borderline friend that already attempt suicide and now suddenly refuse to take is medication for depression?
Sadly- not much. Even doctor's cannot control some people, despite all of our expertise. It is very common for those with borderline personality to make gestures toward self harm. It is also common for friends and family to try and help, often leading to frustration and resentment. A good book for you to read is: codependency no more by melody beattie. Nice of you to care about your friend.
They have to want it. Your friend has to want therapy and medication, and all you can really do is encourage him/her to seek both and engage deeply and meaningfully in the therapeutic process. You simply cannot do it for him/her, much as you'd like to. You can either be his/her friend or therapist. You cannot be both, and even though you want to see him/her get better, trying to be both simply will not work.