User blog comment:Miskos3/Fairy Tail 520: Dragon or Demon, Review/@comment-7418318-20170204161419

I've seen the argument that it is convenient for Natsu to figure out what Irene couldn't spread over several threads, so allow me to answer up here, instead of going to each thread seperately, because I have a theory on the matter:

Natsu figured out how to get rid of the seed - which is the catalyst that causes the transformation - before it could change his body too much. Since I seriously doubt that he'll just stop being a Dragon Slayer now, it would seem that removing the seed won't reverse the changes that have already been done to the body (just like the Dragonification anti-bodies only stopped further progression, without turning the DS back to regular Humans).

So in that light it would have been useless for Irene to try to get rid of her Dragon seed, since the damage was already done. She already was a Dragon, so removing/destroying the seed like Natsu did would have done nothing.

As for how she didn't figure out what Natsu just did prior to her transformation, that's pretty simple: She didn't have as much time as Natsu. If Igneel hadn't stopped the dragonification for Natsu, he would most likely have been a Dragon long before the current epiphany could happen. And on top of that Natsu had the two seeds already trying to destroy each other, which might have futher helped in figuring out how to destroy the seeds.