I doubt there's much you can do. She has her own life to lead and she must plot its direction. If she errs one hopes she learns lessons but people don't always in which case some deeper trouble is at work.

You have no reason so ever to feel bad. I know just saying that is no consolation and won't make you feel better instantly - but here you are on a spiritual forum so presumably you're willing to try. You are not to blame. People own their problems and their actions and while you may hope to influence them it's often best to leave them to their own devices.

I reckon you should back off a bit. Give her time to sort herself out. Find things to distract to keep yourself neutral.

It's distressing to lose a best friend, like someone you can really trust - it's happened to me several times since my teenage...people change, they move away, circumstances change, and I'd hate to lose my current one, my soulmate. So I know how you feel...but not all is lost as once things have settled down she may be back in full. Be there in case she needs you. Her new partner might be acutely jealous of you hence giving you the cold shoulder.

But you aren't to blame for her actions.