My friend is working in Europe. He has some savings (more than Nesab). He saved it for his marriage after returning to his country. One of his friends told him that there is no Zakah in this money so, he did not pay Zakah from 1996 till now. He want to ask if he must pay Zakah on this money or not. If "yes", what he will do in all these previous years. Note that this money is increasing from 1996 till now.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Your friend is obliged to pay the Zakah for all previous years if the amount was equal to the Nisaab (the minimum of one's holdings liable to Zakah) and a lunar year has passed after possessing it. The reason for this ruling is that Zakah belongs to the rights of poor and needy people and others. As a general rule, the rights do not become invalid by the passing of time.
Therefore, your friend should calculate the money he owned in 1996; if it was equal to 85 grams of pure gold or more than he should pay 2.5 percent of total money as Zakah. The same procedure should be applied to the remaining years. We ask Allah to bless upon all of us.
Know that Zakah is obliged only if the money reaches Nisaab, one lunar year has passed after owning it and the value of such money is increasable. Kinds of wealth that are increasable include gold, silver, cash and assests, i.e. they are subject to Zakah, whether they are kept aside for marriage or building a house.
Allah knows best.