Tallulah Willis allegedly checked into rehab last month.
The 20-year-old blogger, whose mother sought help for ''stress and exhaustion'' in 2012 and drug abuse in the 1980s, secretly began treatment at the famous facility after Demi convinced her to seek help.
The insider told Star magazine: ''Demi had been shocked when Tallulah confessed to her. But she knew enough not to judge. She knew her daughter needed her and she was glad she could be there to help.''
Bruce reportedly threatened to cut Tallulah off financially in May after his 23-year-old daughter Scout informed him she saw her sister using drugs at a party.
Meanwhile, Tallulah recently revealed she has always felt ''ugly'' and was diagnosed with body dysmorphia when she was 13.
She said previously: ''I was trapped in this body. I started starving myself and losing a bunch of weight and I got down to like 95 pounds. When I lost my curves and when my boobs shrivelled up into like nothing and I had no shape. It put me in even more of a cage.''
Demi and Bruce, who also have daughter, Rumer, 25, split in 1998 after 11 years of marriage.
The 'Die Hard' actor also has daughters Mabel, two, and Evelyn, three months, with wife Emma Heming.