Who was the boyfriend she calls William?

In Brave, her book, she says when her mom decided to skip town with her boyfriend, she “left” 15yo Rose in the “care” of Rose’s 20yo boyfriend. She describes him as the spoiled son of a Hollywood insider. She describes him as abusive, and a drug user. They lived at his family’s mansion in Beverly Hills.

She says he was very dependent on his mother and calls him a mama’s boy. When his mother suddenly died he basically went nuts, using heavier drugs, and very frequently. She says they moved into his mother's Bel Air house three days after her death.

She says his father had a British accent.

Any idea who this guy is supposed to be? Who were his parents?