[riceflick]

"She's crying!... er, I'm crying... YOU HAVE TO STOP"
"... Thanks, gran I'll finally take notice of youuu"

I think Sandi knows Derek's a phony, but decided to go along with Derek's act because she's generally supportive. Like when a friend tells an unfunny joke and you laugh anyway to keep them from looking like a complete fool in front of everyone.

but Derek also knows he's a phony, so imagine his surprise when Sandi Bogle began acting like she was possessed by the deceased Granny Acorah. Naturally, Derek had to pretend Sandi's possession was legit or else his entire charade crumbles.