Plot hole: Why would Sian let Griff into the shower room if she broke up with him the night before?
Plot hole: The shower room door has a lock, that as shown in the denouement the killer never closed - making it fairly obvious that the victim did not lock herself inside the room to kill herself. Another minor but still significant problem with the stratagem used; the killer shot the girl in the chest; even if they didn't do any gunshot residual test on her hand, the trajectory of the bullet still would have proven the impossibility of a suicide.
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.