Lidl had protested to her office as well as to the Advertising Standards Authority, over the full-page ad in an Irish national newspaper, pointing out it compared a Tesco packet of 28 nappies for E4.43 with a 62-nappy Lidl pack for E11.39.
The E7 difference, said Lidl, had helped skew the overall price comparison.
Commenting on the controversy, Foley said: "I am very disappointed that any retailer would cause shoppers confusion in this way, and I believe this ad caused confusion.
She added: "I know you can't spoonfeed consumers, but I think it is quite legitimate when you present two columns of prices for comparison to think you are comparing like with like."
Earlier, a Tesco spokesman had claimed the Advertising Standards Authority would have the final say.