Well @Oldbutstillgotit, my suspicion is that he wrote the biography the same way that the other 6/7 were written about H&M as well as how other biographies in general are done.
By going over newspaper, tv, magazine reports etc. and possibly going back to sources to re-hash the validity of the stories. And perhaps, hearing the story once again, but with a different take on that same story. eg. it can't have been hard to get info on their first date from Soho house staff. That date must have been the talk of the locker rooms for weeks.
He probably spoke to her friends who would have trusted him to tell the truth and not twist it. I remember the actress Catherine McPhee on Jeremy Vine, saying she did not really know MM and they were together for a short while (either theatre or acting school or something), but she left out the fact that her husband is so close to H that he organised for the rental of the Canada house. She only revealed this in the US, after H&M move. Maybe she only trusted certain outlets or she did not want to be hounded.
Maybe he also got some of the info from the same 'palace sources' as the rags.
I also think he made his own conclusions or guesses on some items. I suspect that he went to the safari camp and asked something along the lines of where is the loo, and when told it's quarter mile a away, did his own maths and peppered it, so the book is not too boring and stifled, he did after all say that he wants ppl to see the romantic side. Either himself came up with that romanticism or the sources did, could be a mix of both. Another example is when he heard when Mike Tindall said they have a family WhatsApp group, looks like he/ or his source concluded that everyone is in it, including PC, which it turns out he is not.
One of my problems is how H&M are constantly pre-judged as they will do bad things. So I am not too comfortable to do that a bit like 'minority report' style.
Interestingly, Ingrid Seward just recently wrote a biography on PP and not one person said anything about a collaborate. Or 'fingerprints all over the book'.
From the get go both H&M and Omid denied any collaboration on the book, before it even came out for distribution.