If she thinks she may be anaemic she should probably go to a doctor. I don't think that much blood is loss either, something like 40mL over a few days. It's just the egg and endometrium isn't it?
I have a very rich diet in iron(huge fan of red meat,i eat plenty of dark leafy greens spinach is my favorite! ) and i am still deficient, that is why i recommended she get her Ferritin checked because i have low levels of that. Other than that i agree with Austinite.
If you're iron deficient it generally means your diet is lacking. Supplementing iron should only be used in cases where Ferroportin fails to pull iron stores out of cells for metabolism, or if the liver is not producing enough hepcidin (the master iron regulator). These are somewhat rare cases.
Eat more iron rich foods as DPR suggested.
Iron deficiency anemia is very commonly caused by heavy menstruation. Internal bleeding, as caused by ulcers, cancer, hemorrhoids, or regular aspirin use, is the most common cause.
Ok.. what's that got to do with the price of tea?
OP mentioned his girlfriend had a very heavy period. So she is a candidate for iron supplementation.
Well, everyone is a candidate. But I see your point. It's important to note that the majority of malnutritioned humans are women. Supplementing iron without confirmation of deficiency cause is unnecessary. That will only lead to elevated RBC production if she is not really deficient.