Do we call a person from China "a Chinese"?
For example, can I say: I have a friend, he is a Chinese?
He is a Chinese is okay, just like She is a Portuguese is okay. But probably most native English speakers prefer to say He's Chinese and She's Portuguese.
See the Wikitionary usage note about a Chinese:
As with all nouns formed from -ese, the countable singular form ("I am a Chinese") is uncommon and often taken as incorrect, although it is rather frequent in East Asia as a translation for the demonyms written 中国人 in Chinese characters or Japanese kanji.
To describe your friend from China, you would say
He is Chinese.
He is a Chinese (person)
although the first gets used more often.
More formally, the Immigration authorities might say
He is a Chinese national.
he has a Chinese passport
if he was the only Chinese in your school, you might say
He is the Chinese at our school.