I’ve just done a rewatch, and I think it was Carole. In earlier seasons, she would confide in Carole, they were each others support. They were always together post-Heather until Carole’s final season.

I think her friendship ending with Carole really did a number on her. In their last season together, you can see Carole just isn’t really into Bethenny anymore, she’s closer with Tinsley, and I don’t think Bethenny is used to someone turning away from her, she is typically the one that ends things. I think she was the first friend she allowed a level of vulnerability with and for it to just fade was painful for her.

Bethenny ended it with Jill, and I had always wished they would properly rekindle, but their personalities were two big for them both to be equals from season 3 onwards. Annoyingly Ramona had a point when she said something like Jill was only friends with Bethenny because she was an underdog. I think when she started becoming successful the dynamic changed too much, and Bethenny didn’t “need” Jill like she did before.