Police said the boyfriend and the daughter, whose identities were not released, were in a physical dispute sometime around 6 p.m. Saturday at the daughter's apartment in the 2800 block of East Valley Boulevard in West Covina.
The mother, 40-year-old Christine Marie Rodriguez, intervened by stabbing the 24-year-old boyfriend once in the back, police said.
Police were called and arrested Rodriguez for attempted murder. She was held at the West Covina Police Department on $1 million bail.
The stabbing victim was taken to Los Angeles County-USC Medical Center, where he was listed in stable condition.
