Asked a similar question earlier but I think I have a better idea of what I really need to know now
Original post
https://www.reddit.com/r/BPD/comments/ygl3j9/how_do_i_help_my_bpd_friend/?utm_source=share&amp;utm_medium=android_app&amp;utm_name=androidcss&amp;utm_term=1&amp;utm_content=share_button

I realized after I made that post that I was thinking about it the wrong way. As far as I can tell splitting is pretty much unavoidable, so how do I help him afterwards? The depression always hits him really hard after he "wakes up", esp if he's said anything particularly nasty, so how do I make sure he knows that I don't hold it against him? That I understand (as best as someone who hasn't experienced it themselves can) what he's going through and I'm not going to walk away from our friendship no matter what. His mental state afterwards is almost worse than the splitting itself bc afterwards he thinks he deserves everything that he's been through and says stuff like how he doesn't deserve me and I hate that so much.
I hate his family for how much they've affected his self-esteem and I hate that I can't make him see that he's just as deserving of love as anyone else. I hate that there's been just enough shitty people in his life that I can't convince him that most of the demons are in his head. 
I don't like to say I forgive him bc to me that implies that I blame him for what he says, but what else can I say?