Re: Question about ending
If you are talking about the full ending, then the closest thing to someone turning bad is not Cassandra but Solas. Solas is set up as the protagonist for the next game.
Cassandra can become one of several different things depending on your choices, but none of them would I consider bad or against the Inquisitor. She can be romanced, made the new Divine, and or train up a new Seeker order. I think there may be a few more differences available but these are the ones I know about.
See spoiler tag if you want to know or simply read the Wiki: http://dragonage.wikia.com/wiki/Cassandra_Pentaghast
As far as the game goes....lol, you can spoil some of the ending, but there is still a bunch to roll play and find out even if you read about it, there is lots to do to make a fun play. So many things can vary depending on how you play and how your character acts to different people, tell him to just get into the game and become his character while playing and it will all work out.
