Kelly isn't blameless, but I guess the fact that she's on trial for Seb's murder and facing serious jail time is what is driving this "sympathy" thing (since we know she is not responsible for his death).
I remember Simon and Kelly had set up a date and he did not show. Not sure why...maybe the drug mess he was involved in or something to do with Peter. Simon had a lot going on at the time. Anyway, Kelly was quite angry about it.