Sudden pulling to the left when gas is applied, then when the pedal is eased it returns to normal.

my friend has this vehicle. there is no noise/clunking or other. the problem is: when he is driving along and steps on the gas, the car suddenly pulls to the left.(only). when he lets off the gas, it returns back to normal. it only does this when he steps on the gas.it just started a few days ago but hes worried it could cause him to get in an accident. he tried it in both front and all whenn positions, and it still pulls in both, does anyone have any ideas of what may be causing this. so i can tell him. he hasn't taken it to any dealership, but he wants a friend mechanic to have a look. so he would like some idea of what to look for. thks.