it might not be one ingredient, but a combination of things.
send your gf to a doc for an allergy test, if she hsn't been recently. you can develop allergies throughout your life; you aren't necessarily born with them.
i had some strange allergic rashes when i was young, and it seemed to occur when i combined chocolate with champagne. my doc guessed it might have also had to do with hormones.
tiramisu is often coated in cocoa or espresso powder, so look into those as possible clues, having ruled out eggs and dairy.
my sister is severely allergic to shrimp. we found out when she was about 10 or 11 years old. before that, she ate breaded shrimp or shrimp cocktail with the rest of us whenever mom treated us to it.
well, one night during a dinner of shrimp, my sis started to have a bad reaction and couldn't breathe. as her tongue and throat swelled, she tried to tell our parents that something was wrong. of course, another sister and i started to imitate her funny "fat-tongued" speach, also complaing to our parents and getting louder and rowdier all the while. dad put down the law and we all shut up, until my poor sis turned pruple and passed out in her plate. fortunately, my dad was a world war 2 medic and was able to stabilize her until they got her to the hospital.
fun times with allergies...