Why laptop reboots after electrostatic discharge
Actually a typical modern "isolated" charger schematic looks probably more like this: -



Note the 2.2nF capacitor connecting output terminals back to the rectified non-isolated supply. It is present to cut-down on EMC on the output and will of course be a pretty good path for a discharge of current to the AC wiring and hence earth.


  What is the path for dicharge current? Low voltage part seems to be
  isolated from power lines.


Not on a modern power supply - static discharges will fly through the aforementioned capacitor.