grammar - About は particle and the 'contrast' it implies

I started learning Japanese about 2 months ago and today, while talking with a friend of mine who has been studying Japanese for about two years, he gave me this example scenario about は particle and the 'contrast' it creates; and it confused the hell out of me:

Imagine a group of people conversing. One of them asks the entire group what are their nationalities. Half Japanese one says he is Japanese by saying: 私は日【に】本【ほん】人【じん】です。And right after him another Japanese guy says the exact thing too. The half Japanese guy feels offended.

Now, the reason the second guy's answer offended the first guy is, he implied that he is more Japanese than the other one. If he didn't wanted to imply that, he could just use the も particle instead of は. Because after は, you state something that is lacking in the previous 'topic'.

But I read in an online source that the が particle puts the focus on the the word it attaches (subject) while は puts the emphasis on the part that comes after it. And that source, as an example, stated that if someone said これはぺンです while holding a book, you would point to a real pen and then say いいえ、これがペンです Which would mean: No, this is a pen.

Is the example my friend gave me about は particle correct? Shouldn't there be が instead of は for the second guy to mean "I'm the Japanese one!"?

Answer

Native Japanese speaker here.

Your friend's scenario makes no sense, period. Your own understanding of 「は」 indeed makes far more sense.

The contrastive 「は」 works like this:

Let us suppose you have recently been to three new restaurants and you have a different impression of each. You might say:

「Restaurant A はとてもよかった。B はまあまあだった。でも C はよくなかった。」

meaning:

"Restaurant A was very good. B was okay. C, however, was no good."

That should sound self-explanatory, no? That is comparison and contrast. It is made possible only by using the contrastive「は」.

Thus, in your friend's example, it would have been correct if the second person had said:

「私も日本人です。」 using a 「も」.

Using a 「は」 there is not something a native speaker would intuitively do regardless of his intention. Had he actually meant to offend the first person, he would have said:

「私が日本人です。」 (I'm the Japanese.)

That sentence clearly implies that the first guy who said he was Japanese is not really Japanese.

(The problem, however, is that quite a few Japanese-learners mistakenly use 「が」 as if it were the default topic-marker. SE is no exception.)

What you stated about the sentences regarding pen vs. book is, of course, correct.

Is the example my friend gave me about は particle correct?

No, not at all. You already know better than he does regarding this matter.

Shouldn't there be が instead of は for the second guy to mean "I'm the Japanese one!"?

Only if the second guy actually intended to offend the first or to make a joke (about the first guy being only half-Japanese.)

As I stated above, if the second guy had neither of the two intentions described above, he would have most naturally used 「も」.