Rebecca Vowles, 47, hurled a tirade of foul-mouthed abuse at Samantha Newby-Vincent before the toilet attack at the exclusive marina in Dorset.
Vowles was alleged to have bashed down a locked cubicle door and punched her terrified love rival in the head before being dragged off her by ex Max Walker.
But the glamorous blonde defendant said she couldn't have carried out the attack as it might have broken her immaculate nails.
She said she also placed too much value on her expensive rings to risk damaging them.
Vowles said: "I've never bashed a door down in my life — I love my nails and rings too much to do that."
Instead, she claimed Miss Newby-Vincent, 43, faked the attack by creating a loud bang against the hand dryer and screaming out.
But magistrates found Vowles guilty of assault after being shown a video which partially recorded the scrap.
She will be sentenced next month.
The court previously heard Vowles and Walker acrimoniously split in August 2012.
Mr Walker has been with Miss Newby-Vincent, a marketing manager and horse rider, since March 2018.
On June 10 last year Mr Walker and Miss Newby-Vincent had been at the Salterns Marina Hotel in Poole Harbour, Dorset, for the prize-giving ceremony for a powerboat race he had come third in that day.
I wear the same rings every day – I never take them off – and if I punched someone with them they would slice their face up.
Vowles was also there and when she saw the two drinking on the garden terrace she made a beeline for their table and hurled abuse at Miss Newby-Vincent, calling her a "fat c**t".
Mr Walker, 51, later received a text from his partner asking for help and he rushed in to the toilets with his mobile phone on record.
Defending herself, Vowles denied punching Miss Newby-Vincent and claimed the whole thing had been "a set up".
Vowles, of Canford Cliffs, Poole, said: "Sam looked at me, smirked and hit her hand on a hand-dryer before screaming.
"I never hit her."
"I had never met her before and had no reason to hit her.
"I wear the same rings every day – I never take them off – and if I punched someone with them they would slice their face up.
"There is no blood in the images of her injuries and it looks more like a mosquito bite to me."
Speaking of Vowles, magistrate Amanda Kaye said: "We did not find her evidence to be clear or convincing and her version of events was inconsistent.
"We therefore find her guilty."
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