There are 2 people in my life with BPD, one being my partner and the other a friend. There are a couple topics between us all that nobody can mention because it causes a split in someone but recent events have meant some of said topics really need to be addressed. One of the bigger things is something I am not too involved with and have been keeping back from because it's not my place to intervene and my partner has tried talking to the friend about it but it causes him to split which then causes my partner to split and the topic never gets discussed. There have been multiple attempts but all end in the same way and it can't go on like this. If the topic is ignored it will be so much worse and it will fester for a year, making all involved miserable and anxious.   
Does anyone have any advice on how such topics can be approached carefully? Reassurance seems to not be effective and the reason the splits happen I think is because people's feelings on the subject are unsure and scared because it affects the future.