My friend accidentaly took too many Advil (ibuprofen) three days ago and ever since then has had severe stabbing pain in her abdoman. What could this be?

Ulcer. It could be acute gastric ulcer. Your friend need to go to er.
Pain. This should be evaluated by a physician. If related to the advil (ibuprofen), (ibuprofen) this could be gastritis or an ulcer. There's also a possibility this has nothing to do with the advil (ibuprofen).