Later that day when she was told Mr Nisbet had died, she asked what time it happened, thinking it must've been about 4.20am.
She spoke to Milner that night who told her she had dozed on the couch, before waking, going to bedroom, turning on the lights and finding Mr Nisbet dead in bed at 5.50am.
Mrs Carey told the jury she replied: "Helen, you couldn't have turned the lights on at 5.50am, the lights were on at 4.20am.''
Milner "ignored her'' and changed the topic, she said.
Under cross examination by defence counsel Rupert Glover, Mrs Carey accepted the fence was taller than she was, but stressed she could see her neighbour's property clearly from the aviaries in her backyard.
Mr Glover suggested the bedroom had thick drapes which meant she couldn't have been able to tell if the lights were on. Mrs Carey said that was wrong.
Milner denies murdering 47-year-old Mr Nisbet, her second husband, by giving him a fatal overdose of the antihistamine and sedative Phenergan, and possibly finishing him off with a pillow over his face.
She also denies attempting to kill him twice on April 15, 2009.
Mrs Carey told the court she'd been a close friend of Milner's and popped over for coffee and chats. She said that during one visit Milner and one of her sons were discussing what to put in Mr Nisbet's water bottle to drug him so he would drive over Otira Gorge in the Arthur's Pass while on his delivery run.
"I said 'don't be so stupid, nobody is worth going to jail for','' Mrs Carey said.
"I just thought she was being silly and I told her not to be so stupid.
The Crown says Milner was unhappy in her marriage and motivated to murder by the prospect of cashing in the $250,000 life insurance policy.
She plotted the best ways to kill her husband - buying drugs under false names, asking friends for views on poisoning methods and even offering to pay $20,000 for a hit man to kill Mr Nisbet, it is alleged.
The trial, before Justice David Gendall, continues.