Monika Fourie said she was going to make a cup of tea after walking in on Hannah Stokes sitting with estranged husband Wouter in the sitting room of the marital home, the jury heard.
Ms Stokes suffered severe burns and a perforated eardrum in the attack, the jury was told.
Fourie, of Olympic Way, Glenholt, denies causing grievous bodily harm with intent but admits the lesser charge of causing grievous bodily harm without intent.
Jo Martin, opening the prosecution, said Fourie had been "eerily calm" when she asked her Royal Marine husband Wouter to introduce her to Ms Stokes.
She told the court Fourie asked her husband to go upstairs before later approaching Ms Stokes with a cup of hot water and throwing it over her.
Mr Fourie then came downstairs to witness Fourie pouring a kettle over her, Miss Martin told the court.
Judge Paul Darlow commented Fourie had been suffering from anxiety and depression.