Two years back a good friend of mine got in an accident where a nail got stuck in her waist. It was removed from her body, but after two weeks she got iron poisoning and fell into a coma for a week. Last week she again fainted. Doctors say it might be iron poisoning. Can iron poisoning return? Was it still in her body? She wasn't on any supplements or medication, so I'm not sure what could have caused this. Though it might be iron overload (hemochromatosis) from what her symptoms show.
We can't diagnose your friend on the internet; only her doctors can do that. Guesses are good in game shows; in medicine, it helps to have more to base a diagnosis on.
You are correct that acute iron toxicity (for example, from children eating iron-containing vitamins because they are candy-like) can result in coma. But to my knowledge (and I have seen and treated many metallic foreign bodies), iron toxicity from such an exposure would be unlikely.
Aside from ingestion, the two most common reasons for iron overload are
1) repeated transfusions required for various anemias (where red blood cells are destroyed by the body and need to be replaced), or when bone marrow has stopped functioning properly and RBC's must be supplied; this is called secondary hemochromatosis,
2) iron-overload disorders owing to genetic misregulation of iron absorption, referred to as hereditary hemochromatosis (or primary hemochromatosis).
Primary hemochromatosis can be treated by removing blood periodically, forcing the body to use up the excess iron, or by chelation therapy. So, untreated, yes, iron overload from this source is permanent. Acute iron toxicity can be treated with chelating agents is severe enough.
While hemochromatosis is not rare, fainting is very common (so common it is difficult to estimate it's occurrence due to underreporting); rough estimates are that at least 35% of the population will have at least one episode in their lifetime. There is a bimodal distribution, with the first peak around the late teens (one-third of medical students report at least one syncopal episode) and the second starting around the seventies.
Again, numbers are hard to obtain for fainting (syncope), but the most common cause is vasovagal (fear, pain, prolonged standing, etc.), followed by cardiac causes.
Although fainting is common, the only real way to determine its probable cause is to see a medical professional, who can gather the information necessary to make a diagnosis.