Lindsay Lohan's Former Assistant 'Fully Cooperating' With Prosecutors, Will Be Key Witness Against Her

Lindsay Lohan's former assistant, Gavin Doyle, has been cooperating with prosecutors handling the troubled starlet's lying to cops case, and he’s expected to be a key witness at the trial, RadarOnline.com is exclusively reporting.

"Gavin was formally subpoenaed by prosecutors last month. He was the passenger in the Porsche Lindsay was driving when the car accident happened last summer," a source close to the former assistant tells us.

"Gavin has been cooperating with prosecutors and will testify that Lindsay was driving the car that day. He will tell the truth, because he has nothing to hide. He has been extremely concerned for her health and well-being," the source says.

As RadarOnline.com previously reported, Judge James Dabney slammed Lindsay’s attorney last Friday for his lack of California law knowledge -- branding him “incompetent” during a pretrial hearing for the troubled starlet’s upcoming case.

There is a plea deal on the table for Lohan to go to rehab for 60 days and avoid jail time; however, the 26-year-old Liz & Dick star is refusing to take it because, she vows, she does NOT have a problem with booze or drugs.

According to an interview Doyle did with the New York Times, he recently met with Lohan and Heller to discuss "the car accident related to her probation violation."

Our source says, "Lindsay will likely be stunned at Gavin's willingness to cooperate with the case against her. Gavin just wants Lindsay to get her life together. If Lindsay doesn't reach a plea deal in the case, Gavin will testify against her, and he won't be forced to do it."