first off Dragons, succubus are demons, not physical beings, so she is not one. secondly, do you have any proof she put a spell on him? third, just because she's a satanist doesn't make her evil. i have some really good friends who are satanist and they're the nicest people i've met.
if she did cast a spell, start by protecting yourself, if your ex thinks so too, you should cast a protection spell on him also [or give him a charm or something] then there's a number of spells you could try, but since i haven't heard his side, and this could all be a heartbroken girl wanting her ex back, i'll suggest an anti hex spell or some type of purification. if it is discovered some really strong curse is on him, then i'd suggest spells to break them up, but this other girl would have to be very powerful, and would probably have to repeat the spell she used regularly in order to keep someone who has no true feeling for her.