There is truth in what he says. The point is that magic is forbidden in the Bible. The problem with this is that the Bible is written to the sons of Adam (Passages can be given to support this on request). That means it has no rule over any that are not of the race of Adam. In many fantasy worlds, there are creatures that are not descent from Adam, and even humans of those worlds may not be of Adam's race. Thus, the laws of the Bible cannot be strictly carried thence.
Not, I'm not going to get into the immutable nature of God and those immutable laws that stem from it, but it can be theorized that God could create different laws into different natures(different worlds, if you will). One of these laws could be a magical system. In that sense, I am not opposed to magic. However, I would agree that no son of Adam could use magic without being a heinous villain.
As far as demonic content, realize that not every story is comprised of good guys. Some people play the villains and some are of demonic influence. Now, as MD says, most of us choose not to get into technicalities for sake of, well, I personally, for the sake of honoring God. All things must be done to His glory, including villainy (I still love how awkward that sounds.) But realize that just as most stories have heroes, most stories have villains, and these may or may not be in league with demons. In that sense, yes, we do allow demonic content.
Now, on the flip side, I would say that never could a character in league with demons be anything but a villain of awful qualities.
So, I hope that flushes out the answer and give you some things to think about. Still, the Bible says that if you cannot do something in good conscience, then you would be sinning to do it anyway. So, if the Holy Spirit prompts you not to do so, I understand and respect that. Realize, though, that not everyone holds the same view on fiction as you do and respect the leading of the Holy Spirit in their hearts as well.