According to the mags Deano mixes Stacey's drink with drugs to try and cheer her up.
Because I know when I'm miserable being drugged and put in danger of attack, rape or murder always makes me feel much better.
I think they may go the route of his intentions being good so everything's fine. It's not as bad as if he'd planned to attack her or take advantage, but it's still a stupid thing to do.