French viscount and fashion designer Arthur de Soultrait said that the socialite had not been amused when his friend - who was driving them to a party - waved the pretend pistol at members of the paparazzi on Saturday.
de Soultrait told AFP: "The car was being driven by a friend of mine, whom Pippa had only met a few hours before... As a joke, my friend brandished a toy pistol in their direction.
"Obviously Philippa had no idea that he was going to do that and told him to stop immediately. She did not find it funny. I regret enormously that this incident happened, and especially that Pippa has been subject to the subsequent attention through no fault of her own.
"I have apologised to her for this."
de Soultrait added that the group had a toy gun in the car with them because they were travelling to a costume party with a theme of "18th century decadence".
He also criticised "constant and dangerous harassment" of Middleton - younger sister to the Duchess of Cambridge - by paparazzi during her time in Paris.
de Soultrait's brother Marcy told the British press earlier in the week that he felt the incident had been "blown all out of proportion" and claimed that Middleton thought it was "a joke and a funny game".
A Paris police spokesperson denied that the incident was being investigated or that any charges had been filed, and additionally pointed out that they had been overwhelmed with calls from British media about the story.
The incident comes amid a police clamp down on weapon usage in France, following a number of gun-related murders in the south west.
The offence of carrying a live weapon attracts a prison sentence of up to seven years in the country, and anyone found guilty of brandishing an imitation weapon could be imprisoned for up to two years.