Claims to be Tiawanese / Japanese
Joint Taiwanese / Japanese heritage is a possibility. The Qing Dynasty of China ceded Taiwan to Japan in the Treaty of Shimonoseki in 1895; however things have changed somewhat since then.
Today, 57% of people on Taiwan consider themselves to be Taiwanese, 23% Chinese and 20% both Chinese and Taiwanese - so for someone to state they are Chinese / Nippon is very, very unusual.
Japanese usually state they are Nippon, too, in my experience (unless this chick is so culturally sensitive that she believes that some may not understand what Nippon means).
I'd say she's almost certainly Chinese.
Ask her if she is 'more Chinese or Japanese' for a bit of fun
Anata wa yori ōku no chūgokugo matawa nihongodesu ka?