"It's stressing him out to no end," says the source, noting that so far, the actor, 43, "wants nothing to do with it."
The Clueless star, who dated Kutcher after meeting on the set of 2003's Just Married, was 32 when she was found dead in her L.A. home in 2009, reportedly due to pneumonia, anemia and drug intoxication. But after her husband, Simon Monjack, died five months later under similar circumstances, questions have persisted about the mysterious tragedy.
"The doc is going to look for clues as to why it all ended like this, and producers want Ashton to tell his side of the story," dishes the source. "The thinking is he may have little choice but to participate because he had a front-row seat to Brittany's life — and her fans want him to shed light on what really happened."
"Overall, Ashton's a positive guy who counts himself very blessed for all the good fortune and success he's achieved," a source previously told OK!, adding that Kutcher is a "sensitive individual who takes it badly when tragedy occurs."
"[T]his awful news about Tanya has hit him especially hard," the source added.
Roberts was initially confirmed to have died on January 5 at the age of 65 years from a urinary tract infection. However, her death was announced prematurely — around 24 hours. The news stunned Kutcher and others who knew the former Bond Girl.
"Now that Tanya's truly gone, it's brought up a lot of painful memories and emotions," the source said of Kutcher dealing with the loss of his friend.
However, these are not the only times Kutcher has lost someone close to him. In 2001, his then-girlfriend Ashley Ellerin, 22, was brutally murdered in her L.A. apartment by a serial killer.
"It's an experience that stays with you for a lifetime," an insider had said of the tragic incident. "It's scarring."
The source added, "The word is, he's told friends he still has terrible dreams where he sees Ashley lying in a pool of blood, and he wakes up sweating and crying."