Larry Williams - a prominent stock trader based in Australia while he faces U.S. tax evasion charges - told Sydney's The Daily Telegraph about the Hollywood couple.
"We've known about their troubles for a while but it's always a very difficult thing in life when these things happen," Williams was quoted as saying.
The pair got together on the set of 2005's "Brokeback Mountain," in which they co-starred as husband and wife. They later settled in New York, and had a daughter, Matilda, born October 2005.
"I know Heath and Michelle still care about each other deeply and are very committed to being great parents to their daughter. ... I feel tremendously for her, and for him and hope they will find what they want in life," Williams said.
A spokeswoman for the couple, Mara Buxbaum, had no comment on the split.
Ledger, 28, and Williams, 27, received Oscar nominations for their performances in "Brokeback Mountain," directed by Ang Lee.
Ledger, who hails from Australia, will next be seen as the Joker in "The Dark Knight," the sequel to 2005's "Batman Begins." The film, which stars Christian Bale, is scheduled for release next year.
Williams, an alumna of TV's "Dawson's Creek," co-stars with Ledger and Bale in the Bob Dylan biopic "I'm Not There," slated for limited release Nov. 21.