The letter was from Meghan to her father after the wedding trying - it was an attempt to address the issues she felt needed to be dealt with after the wedding debacle.
We know a lot about it because it was part of one of the court cases Meghan brought against the publishers of the DM/MoS.
Jason Knauf testified that she had sent him the copy of the letter before posting it to check the content in case it leaked. He testified that she had said she was going to use the term ‘Daddy’ in the letter to refer to Thomas because in the event that the letter was leaked that would ‘tug at the heartstrings’.
The ‘five friends’ were never named but were referred to in an article by People magazine as sources who were talking on Meghan’s behalf about her relationship with her father - the article referred to a letter she had sent him - the article painted him in a bad light via these five friends who it was implied had seen this letter.
In the court case the defendants wanted to know the identity of the five friends so they could could provide testimony about the letter but the judge denied that so we never found out who they were.