Pattinson and Witherspoon co-starred together in 2011's "Water for Elephants."
Pattinson has been staying at Witherspoon's vacation home since news broke last week that Stewart, his longtime girlfriend and "Twilight" leading lady, had an affair with film director Rupert Sanders, who is married with two children.
It is unclear whether Witherspoon, who is pregnant, and her husband and two kids are also at the Ojai estate or if Pattison is there by himself.
A source close to Pattinson and Stewart told UsMagazine.com the 26-year-old actor is "a total mess."
"He's questioning everything," the insider said.
Stewart, meanwhile, is "inconsolable" and has been trying to reach Pattinson by text message and phone after publicly declaring her love for him and begging for his forgiveness last week.
"She's dying to save the relationship. It's the only thing she cares about," a source close to the 22-year-old actress told the magazine.