Alright, I'll try to post something soon.
Edit: Finished my post. I understand that illusions can be one of those hit-or-miss powers a lot of GM's might not like because it relies on people strengthening their minds, which unlike the body isn't something that can be measured. The easiest way to break an illusion two methods: The first is to simply ignore it. A sharp mind can point out inconsistencies and flaws that might let them know that an illusion has been cast on them, and those who are really perspective or even better, illusionists themselves could even clearly see what is an illusion and what isn't. The other is simply trying to "interact" with the illusion physically, since illusions are all in your head and don't have physical forms.
What Sweet Demon did however is trickier because she's relying on the sedative to have put Dreadlock into a sort of sleepy, quasi-dreamlike state, thus it'd be even harder for Dreadlock to discern what's real and what's not. As I mentioned in the post itself, even if Dreadlock thinks it's all a dream that alone wouldn't break the illusion. She'd have to really question Sweet Demon and figure out who she is and what she can do before she can realize that this may all be an illusion, unless someone else can wake her up or get inside Dreadlock's head too.