I've never used a flat iron, but I've had no problems with a curling iron overseas (230 VAC). My curling iron was purchased in the US about 5 years ago and is rated for dual voltage. When I was shopping, I found all of the curling irons at the store were dual voltage. A coworker's wife burned up the motor in her hair dryer in Malaysia though.

Let your friend know that the curling iron will get hotter, so she needs to turn it to a lower setting to avoid burning her hair.