There is no option for time zone within Group Policy. However you can use Group Policy to execute a login script which may include the command to change the time zone to Eastern.
Sample of the contents of a login script:
CONTROL.EXE TIMEDATE.CPL,,/Z Eastern Standard Time
If that is the case, create a copy of this script and change it to:
CONTROL.EXE TIMEDATE.CPL,,/Z Pacific Standard Time
Create a new GPO executing this login script to that user’s computer.