I have a friend who has an F-2 visa and has been in the US for about 4 years and she has divorce documents(from her country,south africa) from her previous husband who was an F-1 holder at that time.her visa expired a few years ago but she met this guy she wants to marry and he is a citizen.any advice on how she should go about this?

Get married and file a clean strong case immediately.

She is eligible to file and adjust status here in the US without any penalty and waiving the penalty for the visa overstay and (likely) unauthorized employment under section 245(a).

Find an experienced immigration attorney and file soon, immigration is getting more difficult. Good luck.

Thanks