The man accused of murdering mum-of-two Marissa Aldrich held her head under water while he was on the phone, Cambridge Crown Court heard today (Wednesday).
Robert McWhir called his friend Luke Holder, shouting "I'm killing her, I'm killing her".
On the evening of December 21, Mr Holder and his girlfriend Clare Oakley, were at their flat in Cambridge Road, St Neots, with McWhir and Ms Aldrich for pre-Christmas drinks.
McWhir, 26, of Potton Road, St Neots, is charged with the murder of Marissa Aldrich, 29, on December 22, last year, which he denies.
Luke Holder told the court how he arrived at the scene in Love's Farm after phoning McWhir to let him know he had left his tobacco at the flat.
Mr Holder said he, McWhir and Ms Aldrich had been drinking alcohol as well as taking cocaine and cannabis.
He then told the court how McWhir had told him over the phone that he had killed Ms Aldrich so he ran to the scene.
Once Mr Holder reached Love's Farm, the court heard how he wrestled with McWhir and managed to lift Marissa out of the water, but she felt heavy and he believed she was already dead.
Mr Holder said: "I basically got down there, and saw a body in the water, and that d**k head [McWhir] was in there.
"He was on top of her. He was on top of her holding her under the water. I wrestled him to try to get him off her. I managed to get him off her. I was a bit drunk," he said.
"Once I managed to get him off her, I picked her up but I knew it was too late. I then ran down the street screaming, I didn't know what to do. Bobby [McWhir] also ran off."
Mr Holder also told the court how he believed that McWhir was on the phone to him whilst he was killing her.
He told the court: "He picked up his phone half way through. Who does that? Who picks up the phone half way through killing someone?"
Mitigating, David Whittaker asked Mr Holder if he was drunk and if his recollection could be blurred.
Holder replied: "I was very drunk, but I can remember it all. I was in shock more than anything."
The trial continues.