A drunk man who showed up to his friend's house and attacked her son claimed the victim had been "disrespectful".

Brian Denis Mayo "arrived uninvited" to the victim's home on October 26 and went to the sleepout, where his friend's teenage son slept.

Mayo banged on the door before forcing his way in, police prosecutor Sergeant Stephen Butler told the Whanganui District Court on Tuesday. A "struggle ensued", and Mayo grabbed the victim, pushing him around the sleepout. When the victim's mother showed up, Mayo kicked her in the leg. Mayo then broke a window.

"The context of this scenario is that the defendant was in an association or friendship relationship with the complainant and her son," defence lawyer Harete Hipango said.

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They had been friends "over a number of years". Mayo had been drinking homebrews with the female victim earlier in the day, and showed up at her house later with more homebrew bottles.

Ms Hipango said Mayo claimed the teenage victim had been disrespectful to his friend, "and matters got out of hand".

She called it "inappropriate conduct, fuelled by intoxication".

Mayo pleaded guilty to wilful damage and common assault. He has already repaired the damage. Judge David Cameron sentenced him to 40 hours of community work.