Although they didn’t seem to be close friends, Robbie had at least met Paula and knew her socially, he was able to cite things she has said and as was mentioned above, observe the change from someone who didn’t drink much and thought people on cocaine were idiots, to someone who seemingly developed a significant drinking problem and ultimately overdosed on heroin.
RW actually said he thought it was “just” recreational drugs, the distinction being heroin is not viewed by some that way (IMO this distinction doesn’t always work because many cannabis and cocaine users for example develop serious issues, not surprising when you consider what alcohol can do, but it’s a nuance that’s important to some people to try to identify).
Grace Dent seemed to have appeared for just having been a journalist around that sort of time that Paula was at her most famous and hounded by the press and I didn’t get the impression had actually met PY.