Praise be to Allaah.
With regard to the property that is rented out, zakaah must be paid on the rent if it reaches the nisaab (minimum threshold) by itself or when other money is added to it, and one full hijri year has passed since acquiring it. One quarter of one tenth (2.5%) must be given as zakaah.
The year for the rent starts from when the rental contract begins.
Based on that, if your friend receives the rent at the end of the year, he has to pay zakaah on it according to the following calculation: 15,000 x 2.5% = $375.
If he receives the rent in advance, at the beginning of the year, then he spends it and does not save any of it until the end of the year, then he does not have to pay zakaah on it.
If he intends to sell the piece of land that he owns, then he has to pay zakaah on it every time one year passes. He should look at its value and pay one quarter of one tenth of it as zakaah.
So if the land is worth $16,000, he has to pay $400 as zakaah.
But if he does not intend to sell the land; rather he wants it to build on it so that he could live there or rent out the building, then he does not have to pay zakaah on the value of the land.
With regard to his monthly salary, he has to pay zakaah on whatever he saves from it, after one year has passed.
If your friend does not save anything from his salary, he does not have to pay zakaah on it.
If a person is in debt, that does not affect his zakaah, according to the correct scholarly opinion. So he has to pay zakaah on all his wealth that is subject to zakaah when one year has passed, and he should not deduct the debt he owes others from what he has.
For more information, please see the answers to questions no. 47760, 10823, 78807
And Allah knows best.