He installs Internet service for users and he does not know what they use it for

Praise be to Allah.

With regard to the Internet and other things that may be used for permissible and beneficial and useful things, or they may be used for haraam things, it is permissible to sell them to one who you think most likely will not use them for forbidden purposes. 

Shaykh al-Islam (may Allah have mercy on him) said: 

Every garment that it is thought most likely will be used for sinful purposes, it is not permissible to sell it or sew it for the one who will use it for sin and wrongdoing. End quote. 

Sharh al-‘Umdah, 4/386 

Shaykh Ibn ‘Uthaymeen said: 

With regard to every permissible action which you know a person or think it most likely that a person will use it for forbidden purposes, then do not help him in any way. End quote.

Liqa’ al-Baab il-Maftooh, 21/139. 

It is not permissible for you to install Internet service for one who you know for certain or think most likely that he will use it in disobedience to Allah. 

But when you do not know how a person will use the Internet, it does not seem to us that it is forbidden to install the Internet for him, because the basic principle is that the transaction is valid and is not forbidden, and we are not certain or we do not think it most likely that there is anything to indicate that the transaction is forbidden in this case.