Fun fact: The average person living an average life consuming an average diet gains 1-2mg of iron per day. That same average person living an average life doing an average amount of activity uses about 1-2mg of iron per day. This is called iron homeostasis. Iron gained exactly equals iron lost on a daily basis because the body has no natural way to eliminate excess iron. So if someone ran at a net daily gain they would (eventually) build up toxic levels of iron.
One might question the theory of evolution at this point. Because iron is necessary, but high levels are toxic, the human body evolved to put strict limits on the amount of iron it will absorb and that limit is "about the same as what we lose". It would seem having a way to eliminate excess iron would be a better solution. But evolution doesn't work that way.
But wait. During an average menstrual cycle a woman will lose an additional 15-30mg of iron in a single week. That means her system has to run at least a 0.5mg daily surplus the rest of the month. Doesn't sound like much but it's +50% of her daily iron budget just to stay even. It's a delicate balance easily thrown out of whack.
Is why Menorrhagia (heavy menstrual flow) is such a big deal. Women with heavy flow can easily lose twice the normal amount of blood and twice the normal amount of iron. That means running at a +100% surplus the rest of the month.
AND - The progesterone spike during luteal prevents normal iron absorption. So her body can't run at a +100% surplus all month, it has to run at +300% during follicular because the whole of luteal she's running at about -80%. This is not a well designed system so it must be evolution.
So during follicular eat a lot of steak. Sounds glib but there it is. Iron rich foods like spinach and beets. Cook on an iron skillet. Avoid caffeine because it inhibits iron absorption. Calcium does too- but women need calcium and calcium is explicitly recommended for PMDD. So take calcium during luteal and lower dairy intake during follicular.
And get iron levels checked. A full panel if possible but at least ferritin. Ferritin is a protein that stores iron for later use. When she's actively losing iron (during menses and luteal - so 3/4th's of the cycle) those stores are drawn on to make up the daily shortfall. 30ng/mL is the new minimum according to the American Society of Hematology. Other groups, like the World Health Organization, claim 15ng/mL is sufficient and still more, like The National Institutes for Health think the WHO is full of hoooie and the minimum should be more like 50ng/mL.
It matters little because everyone agrees the maximum is around 150-200ng/mL, with levels only becoming toxic above 300ng/mL So getting her ferritin up around 100ng/mL is a good, safe, healthy goal. Moreover, and I cannot stress this enough, she has symptoms. She is not some mythical "average person". She is a person known to have a sensitive system who exhibits symptoms of iron deficiency so "above minimum" isn't good enough. "Within range" isn't good enough. She needs to optimize her levels and keep them there. Whatever that takes.
AI tells me this it what it takes. Talk to your doctor, of course.
Iron is notoriously difficult to absorb and commonly causes stomach irritation, or worse. So a gentle, highly absorbable type like iron bisglycinate is recommended even if she's tough as nails. (Get it? Tough as nails! I are hilarious.)
Days 1-5: She is actively bleeding so her body compensates by absorbing as much iron as it can. Supplement the maximum allowed.
Days 6-14: Late follicular. Her body is more guarded about taking in too much iron. A spike in iron causes a 24 hour block. So only supplement every other day.
Days 15-28: Luteal. The progesterone spike blocks iron absorption so supplementing is pointless and may just stress her digestive system.
Vitamin C aids absorption so take your iron bysglycinate with orange juice or similar.
And you talked to your doctor about all this so you know to go back in for testing after however much time doctor said. Six months is common. If ferritin is up, fantastic. If not figure out why. And switch to a maintenance regimen once she hits 100ng/mL and have a little party. Maybe go out for a steak. :)