"In the last few years, her dark side has really come out," says the source, who is close to both Richie and Barton. "Her friends, like Nicole, want to be supportive but they really can't be around her too much. She's too volatile."
"It makes us sad, but we're also like, 'Pull it together and get help already,'" adds the source.
Barton, who was placed under psychiatric hold after calling police with a health problem July 15, has been dealing with personal issues, according to another source, a close friend of Barton's, who says the actress "alienated herself from a lot of her friends."
Estrangement Begins in 2007
When did Richie and The O.C. star drift apart? Sources say the former friends never had "a falling out" but point to a mutual pal's Memorial Day barbeque in 2007 where Barton fell ill and was taken to the hospital.
"Most of her Hollywood friends have distanced themselves from her [since that party]," says a third source. "After that happened, her friends thought it would be a wake-up call, but it wasn't."
Barton's friendship with Richie changed especially when the reality TV star started a family with Joel Madden.
"Nicole is focused on motherhood and her family," adds the source close to Richie and Barton. "[She] considers [Mischa] a dear friend but she's also a mother who can't be around too much negative, unsafe behavior. Mischa is one of the reasons that Nicole is mostly with her other mom friends these days, not the party girls."
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