Hi there and welcome! I am sorry you have been cut off by your friend. It is confusing and painful when that happens. Many of us here can relate to that experience and know the pain it can cause.
My question is should I reach out and possibly bring up the idea of them having BPD or quietly walk away and let them live undisturbed seeing as my coming back might cause more grief and stress? Reason is they have a life event coming up which I would like to celebrate them for but I'm starting to feel like years of friendship have been unreciprocated and it might be time to walk away
Whether or not to say anything will take some time to think about. We generally don't recommend you tell someone that they may have BPD or that they exhibit BPD like behaviors. It usually does not go over well. Also, it is almost impossible to get someone into treatment if they do not recognize they have a problem. People need to see they have a problem and want to get help for help to work. Here is an article that explains this:
Anosognosia and Getting a "Borderline" into TherapyWhen is this special event? Is it soon? I am wondering how much time we have to think about the best course of action for you and your friend. It is great that you care so much about them and want to help.