Police said Maynard became angry and left the room, returning a little later with a wooden table leg and then started to hit the victim over the head with it three or four times.
Their drinking companion separated the pair and the victim left the house, bleeding from a large wound on the back of his head.
Maynard's lawyer, Peter Stevens, said the attack had been provoked.
It was the night before Anzac Day and Maynard had planned to take the photo of her father, a member of the K Force army contingent which took part in the Korean War, to the dawn parade.
The K Force was a 1500-strong ground force, mainly composed of a field artillery regiment and a transport squadron.
New Zealand's commitment to the Korean War cost the lives of 45 New Zealanders.
Judge Barbara Morris said it was understandable Maynard was upset by her friend spitting on the photo, but it did not justify her behaviour.
She remanded Maynard on bail until July 7 for sentencing.