Demeter, some people make things up. The reason they do this is to 1: Sounds powerful and special 2: Scare people 3: Just having a laugh. This may not apply in your case, but it is often true.
By turning into some sort of 'devil', I would imagine this means a demon, as fallen angels are understood by christians. However, as well as horny devils being unusal, at best, in spiritual life, Demons have been around in spritual history for a lot longer than christianity has.
I really doubt your friend is going to grow horns. I strongly believe that people who go round saying they have practiced black magick are not the best sources of spiritual advice and information. Be well :)