Can someone post a link or an article on Fanpop that contains illegal sites or anyway such things? Will they report him for that, or will he be banned from Fanpop?

it really depends on how illegal it is i suppose.
most users will report the link/article/image/video that is posted, and if the main fanpop guys find out about it you will be banned from the site probably.
if you read the terms and conditions it speaks about this sort of thing, like breaching copyright.
you could look at the link or the link for more information.