Olenna Tyrell did the deed but it was planned along with Littlefinger and Ser Dontos was involved in making sure things went smoothly, not that he knew.
“But if it wasn’t the Kettleblacks and it wasn’t Ser Dontos... you weren’t even in the city, and it couldn’t have been Tyrion...”
“No more guesses, sweetling?”
She shook her head. “I don’t...”
Petyr smiled. “I will wager you that at some point during the evening someone told you that your hair net was crooked and straightened it for you.”
Sansa raised a hand to her mouth. “You cannot mean... she wanted to take me to Highgarden, to marry me to her grandson...”
“Gentle, pious, good-hearted Willas Tyrell. Be grateful you were spared, he would have bored you spitless. The old woman is not boring, though, I’ll grant her that. A fearsome old harridan, and not near as frail as she pretends. When I came to Highgarden to dicker for Margaery’s hand, she let her lord son bluster while she asked pointed questions about Joffrey’s nature. I praised him to the skies, to be sure... whilst my men spread disturbing tales amongst Lord Tyrell’s servants. That is how the game is played.
“I also planted the notion of Ser Loras taking the white. Not that I suggested it, that would have been too crude. But men in my party supplied grisly tales about how the mob had killed Ser Preston Greenfield and raped the Lady Lollys, and slipped a few silvers to Lord Tyrell’s army of singers to sing of Ryam Redwyne, Serwyn of the Mirror Shield, and Prince Aemon the Dragonknight. A harp can be as dangerous as a sword, in the right hands.
“Mace Tyrell actually thought it was his own idea to make Ser Loras’s inclusion in the Kingsguard part of the marriage contract. Who better to protect his daughter than her splendid knightly brother? And it relieved him of the difficult task of trying to find lands and a bride for a third son, never easy, and doubly difficult in Ser Loras’s case.
“Be that as it may. Lady Olenna was not about to let Joff harm her precious darling granddaughter, but unlike her son she also realized that under all his flowers and finery, Ser Loras is as hot-tempered as Jaime Lannister. Toss Joffrey, Margaery, and Loras in a pot, and you’ve got the makings for kingslayer stew. The old woman understood something else as well. Her son was determined to make Margaery a queen, and for that he needed a king... but he did not need Joffrey. We shall have another wedding soon, wait and see. Margaery will marry Tommen. She’ll keep her queenly crown and her maidenhead, neither of which she especially wants, but what does that matter? The great western alliance will be preserved... for a time, at least.”
A Storm of Swords, Sansa VI