Marriage Requirement on the Age

Hi,
With reference to your article above
"The persons marrying must reach the required minimum age: for men that is 22 years and for women 20 years old, except that persons may marry younger with parental permission."
My friend (a foreigner) would like to get married to a Chinese gentleman and she is only 18 years old.
- What kind of document does she require to show parental permission?
- Any procedures would she need to follow?
- Could you please kindly provide any reference website / material?
Thank you in advance for your advice
Regards