There's no magic cure as such for BPD but it is highly treatable. People with BPD who receive the proper treatment (which typically involves a combination of therapy and medication) can show major improvements in under a year. Eventually someone receiving treatment may improve to such a degree that they no longer meet the criteria required to be diagnosed with BPD. This was the case for me.
The fact that your girlfriend was willing to tell you about her diagnosis is a good sign. It shows that she trusts you. The best way you can help her is to be there for her, educate yourself on BPD and how it affects her, and encourage her to seek treatment if she isn't already doing so.
Thanks for your question, I hope this has helped!