If you read Gerritsen’s book. You’ll know that Jane Rizzoli found a man, got married, and even have a baby. Jane and Maura weren’t technically llbffs.
So, whoever proposed to have a TV show based on Gerritsen’s book, must have seen Detective Rizzoli & Dr. Isles as llbffs in another dimention. Then, technicaly, Rizzoli & Isles is a one big fanfiction on TV that updates every week with that disclaimer about basing it from a book.