I study in the US and have been in it for one or two years and then will go back to my own country. Should I pray four or two Raka'h for Asr? My friend told me that I should pray two only because I am considered as a traveler.

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 mentioned that studying in a foreign country without the intention of settlement in that country entitles the person to the permission given to a traveler. This is the opinion of some scholars such as Ibn al-Qayyim , Ibn Utaimin and others.
But, the majority of Muslim scholars are of the opinion that everyone who is on a journey and intends to stay in a place for four days or more is considered as a resident there thus; he should not shorten the prayers and also should not implement the other permissions given to a traveler.
The latter opinion is the correct one, safest and more suitable to become free from the obligations of Sharia.
Allah knows best.