I'm going to guess she obviously has not hit menopause yet correct? Vitamin C DOES help with absorption of iron, I take 1-2g of it everytime I take an iron tab for absorption. Calcium also blocks the absorption, so be wary of that. Also what form of iron specifically is she trying to supplement with? There are a few different types, and some people absorb certain types far better than others, some don't absorb certain types at all. Black stool is a sure sign that it is being absorbed.
Magnesium can help immensely if constipation crops up from supplementing with iron.
Also....if she hasn't hit menopause yet, why try to STOP supplementing with it in the first place? Most women who have iron problems pre menopause need to stay on it since they are going to be losing a large amount of it on a monthly basis anyways.