I was under the impression that "just friends" was only an option for Romantic Interest, not Boy/Girlfriend, Fiance or Husband/Wife. I say that because one of my sims wanted to let a Boyfriend down easy (she cared for him but didn't want to marry him). No matter how many ways I played or re-played it (I re-set at least 5 times, trying to find a good outcome), she never got the "just friends" option. The only thing she got was "Mean > Break Up" which starts an all-out, finger-pointing, angry-face, accusatory argument, leading to negative feelings on both sides.
The only time I've ever gotten the "just friends" option was with sims who were in the Romantic Interest phase. It's a sad break-up but not an angry one.
I wish the more involved relationships could have amicable splits, but I'm not sure it's available.