Is it okay that build and help this person setup his web store? .
The basic principle in sharee’ah is that when Allaah forbids a thing, He forbids its price. Singing and music are haraam and are not permissible, because of the hadeeth narrated by al-Bukhaari, according to which the Prophet (peace and blessings of Allaah be upon him) said: “There will be people among my ummah who will permit adultery, silk, alcohol and musical instruments.” The evidence that singing is haraam has been discussed elsewhere – see questions no. 5011 and 5000.
If it is clear to you that these things are haraam, then it is not permissible to help people with haraam things, because Allaah says (interpretation of the meaning):
“…but do not help one another in sin and transgression”
You have to seek that which is permissible and good, and avoid haraam work or helping others to do that which Allaah has forbidden, because the Prophet (peace and blessings of Allaah be upon him) said: “Allaah is Good and only accepts that which is good. Allaah enjoins upon the believers that which He has enjoined upon the Messengers. Allaah says (interpretation of the meaning):
“O (you) Messengers! Eat of the Tayyibaat [all kinds of Halaal (lawful) foods which Allaah has made lawful (meat of slaughtered eatable animals, milk products, fats, vegetables, fruits)] and do righteous deeds. Verily, I am Well‑Acquainted with what you do”
“O you who believe (in the Oneness of Allaah — Islamic Monotheism)! Eat of the lawful things that We have provided you with, and be grateful to Allaah, if it is indeed He Whom you worship”
Shaykh Dr. Khaalid ibn ‘Ali al-Mushayqih
We advise you not to work with the owners of this website, because the money that you get from developing haraam websites is considered to be haraam money and this is haraam. The Prophet (peace and blessings of Allaah be upon him) said: “There is no flesh that is nourished by haraam things, but the Fire will be more entitled to it.” Narrated by al-Tirmidhi, 558; classed as saheeh by al-Albaani in Saheeh al-Tirmidhi, 501.