The 'Piano man' hitmaker was determined to discover who was harassing 27-year-old Alexa Ray on Facebook and decided to ask his celebrity friend for some advice.
As well as hiring a bodyguard, it is said the 63-year-old musician contacted a Europe-based private-security firm who were recommended by the former Beatles rocker.
Sheryl Flynn, 40, was found naked in Minnesota and charged with stalking the star's daughter for several months.
She sent her over 60 messages, and court records claim some included ''pedophilia, sadistic-sexual behavior, violent physical assaults and murder''.
A source told the New York Post: ''[Alexa Ray's] mother, Christie Brinkley, was terrified that someone would harm her baby. Billy Joel was worried but focused.''
It is reported Sheryl - who spent time in a mental institution in 2010 - is still hopeful of meeting Alexa Ray, and told law enforcement she didn't want to carry out the alleged rape or murder threats during this life.