I don't care about Kubler-Ross. She flat out had a get-together purposely excluding Marley, didn't answer her door for Marley even when she was trying to say the final things she wanted to say, and never even saw Marley before she died. She was selfish and awful.
Some people cannot handle it when someone they love is dying. Marley saw that her friend could not face her dying and she wanted to tell her before it was too late how much she loved her and her little girl.
Marley at first thought Renee was being selfish also but came to the realization that Renee loved her but could not bear to face what was coming and she was reminded of it every time she saw Marley. In the store when Marley was buying all her friends what they wanted, Renee had to leave. She was the only one who was able to say out loud that Marley was very sick and she would soon no longer be with them. She left so she would not spoil Marley's happiness at buying her friends the things they wanted. She felt it would be best if she was not around Marley because she did not feel strong enough to keep up a "happy face" the way their other friends were doing.
It seems selfish to some people but that is just the way it is for many people; they just cannot handle the pain they feel when they are able to realize they will never see their loved one again.
Well stated, queen. And the movie did show that Renee saw Marley after the scene when Marley speaks to her through the closed door. Renee is shown as one of the friends who comes to read & be with her while she is bedridden (like her father does too).