Someone explain it to me

Hi guys!
My Japanese is so poor I know.
I was chatting with a friend normally when suddenly I became unable to understand what was going on.
I understood the literal meaning but, I didn't get it.
The message in romaji:
Watanabe Naomi tteiu geenojin ga suki nandakedo.
Sono hito nan dayo.
What I uneerstood:
I like a celebrity who is called Watanabe Naomi.
What is that person(! (Maybe).
Can you please help me and explain it for me.
Oh yeah, I wrote "Wakaranai" and confused face is it wrong?!
Thank you so much, I appreciate your help!

"Sono hito nan dayo" means "what about her?" or "what is she?".

Without the context, I might be mistaken, but "nan" in "nan dayo" is not the shorten form of "nani" (what), but in this case the shorten form of "nanoda+yo".
So I think that "Sono hito nan dayo." should be written in romaji like "Sono hito nanda yo", which means as you might have already guessed, "That is the person, you know."

I agree with the ...(guest) here... Was there any dialogue before this where she showed you a photo of someone or talked about seeing that person in town by chance, or something like that?

No, we were talking about food in my country. Maybe she sent me the message instead of someone else she was talking to at the same time!
Thank you guys so much for answering.