A transcript of the recording was read to a jury in which the woman repeatedly questioned Andre Nethersole until he admitted acting inappropriately.
Nethersole, 35, of Radstock Crescent, Broughton, Buckinghamshire, was jailed after a trial at Aylesbury Crown Court.
The woman said she would "recommend" such moves to others in her situation.
"I would definitely recommend it and keeping a calm head. Going all blazing doesn't always get you the right results", the mother, who cannot be named for legal reasons, said.
"Have a recording, you never know if it can be used or not."
At an earlier hearing, Nethersole was found guilty of sexually assaulting a 15-year-old girl, inciting her to engage in a sex act, and showing a sexually explicit photo to a second girl, who was also 15.
The court was told how Nethersole made a string of sexual remarks to the two girls, including: "The more I can't have you, the more I want you."
Jailing him for three years and nine months, Judge Catherine Tulk said: "You were clearly fully aware of the girls' ages.
"You have shown no victim empathy or remorse - giving no thought to how your behaviour has affected your victims."
In an impact statement, one of the girls described how she had suffered a "year of hell" since the offences, which took place between 17 and 20 May last year.
Father-of-one Nethersole owns the Eagles Jerk Restaurant and Grill in the Eaglestone area of Milton Keynes.
He has also been given a five-year restraining order, preventing him contacting the victims indirectly or directly.
A secret mobile phone recording by the mother of a victim has helped jail a paedophile restaurant owner who abused two 15-year-old girls.