Should he advise those who frequent pornographic websites?

We advise you to ignore these pornographic websites and not look at them at all. Strive to call people to Islam in your own environment and in your own community. That is best for you. 

A person may be sure of his religious commitment, and he may get in touch with people who run these websites, and gradually the Shaytaan will cause him to fall into haraam, especially since there are so many of these sites and the people who set them up are so evil. So they may gradually lead this sincere person astray and may flood him with their filthy e-mails. 

 

So fear Allah, and avoid all places of doom. Protect your religious commitment from any temptation. Keep yourself busy with that which will benefit you, such as seeking beneficial knowledge and doing righteous deeds. Note that warding off bad things takes priority over bringing benefits.

And Allah knows best.