She was in shock for goodness sake. Anyone who has just received news of someone who was important in their life dying would not be thinking rationally. I remember when I lost someone close I would start randomly telling people in supermarkets about it.

Have you never lost someone close????????????

Angie also told Tiffany David died just now rather than saying last night like she told David and John.
When Tiffany asked Ok what happened? Angie It just happened now
It added to her confusion thinking Guest was dead as he is the only David there at that moment. If Angie had said he died last night I think Tiffany would've realised her mistake quicker. Tiffany when being told David her mind automatically thought David Guest even though Angie didn't say it thats why she told the housemates she said David Guest.