Antonym for "posthumous"
Solution 1:
Ante mortem is more clinical and is a better antonym for post mortem. "Prehumous" is a neologism, but it goes beyond just the opposite and indicates "too early". If there is an actual antonym, it wouldn't be very well known; so, for comprehension, I would just say, "I don't think he published any memoirs during his lifetime." ("Published" is better, because if he didn't write them, they can't be his memoirs.)
Solution 2:
I think "living memoirs" works well in your example (on several levels), but I've up-voted Brian Donovan's answer (nonposthumous) as likely to be more applicable under general circumstances.
From Wikipedia: https://en.wikipedia.org/wiki/The_Posthumous_Memoirs_of_Bras_Cubas
"The Posthumous Memoirs of Bras Cubas (Portuguese: Memorias Posthumas de Braz Cubas, modern spelling Memórias Póstumas de Brás Cubas), often subtitled as the Epitaph of a Small Winner, is a novel by the Brazilian writer Joaquim Maria Machado de Assis.
"Published in 1881, the novel has a unique style of short, erratic chapters shifting in tone and style. Instead of the clear and logical construction of a normal nineteenth-century realist novel, the novel makes use of surreal devices of metaphor and playful narrative construction. It is considered the first romance of the realist movement in Brazil.
"Plot introduction: The novel is narrated by the dead protagonist Brás Cubas, who tells his own life story from beyond the grave, noting his mistakes and failed romances.
"The fact of being already deceased allows Brás Cubas to sharply criticize the Brazilian society and reflect on his own disillusionment, with no sign of remorse or fear of retaliation. Brás Cubas dedicates his book to the first worm that gnawed his cold body: "To the worm who first gnawed on the cold flesh of my corpse, I dedicate with fond remembrance these Posthumous Memoirs" (Portuguese: Ao verme que primeiro roeu as frias carnes do meu cadáver dedico com saudosa lembrança estas Memórias Póstumas). Cubas decides to tell his story starting from the end (the passage of his death, caused by pneumonia), then taking "the greatest leap in this story", proceeding to tell the story of his life since his childhood.
"The novel is also connected to another Machado de Assis work, Quincas Borba, which features a character from the Memoirs (as a secondary character, despite the novel's name), but other works of the author are hinted in chapter titles. It is a novel recalled as a major influence by many post-modern writers, such as John Barth or Donald Barthelme, as well as Brazilian writers in the 20th century."
See also: https://en.wikipedia.org/wiki/Machado_de_Assis
Fascinating!
Solution 3:
Dictionary.com lists nonposthumous as a related form s.v. posthumous, and that would do quite nicely and naturally in your proposed exchange.