I would like to know what is best for my friend?

We both spar together twice every week, maybe three times depending on our other commitments. We spar hard, it isn't like light sparring, we are getting in their and pounding each other. In the pass 3 months, my friend has got 9 concussions, and they have been pretty dam hard. She has receive medical treatment straight after, but she has never had any of the following symptoms you get after a concussion and seems perfectly fine weeks after, like the concussion never happened. She tells everyone she doesn't feel it, and it doesn't hurt her, but I don't know if she puts up a brave front, cause it is very hard to read her, even when we spar.

We are both training to fight in the ring, but all I want to know is can some people have harder skulls then others, cause when I get hit it hurts and I have just about all the following normally symptoms.

She is also much younger then me, does that have anything to do with it. I am 29 and she is 19. Thanks