Is helping a neighbor connect to the internet by running a cable through one's home and assisting with the setup a cause for sin if the neighbor uses it to disobey Allah, bearing in mind that the internet can also be used for beneficial things?

The basic principle is that a Muslim should benefit his brothers and assist them in righteousness and piety, and this is even more emphasized for a neighbor. However, cooperation in sin and aggression is not permissible. If it is highly probable that the neighbor will use the assistance for something forbidden, then it is not permissible to help him. If that has already occurred, then repentance is obligatory, and he must be informed that it is not permissible to use it in what displeases Allah, and the wire should not be allowed to pass over the house if it is in violation.