https://www.pixiv.net/artworks/71375545

One of my favorites. The dialogue is hilarious, probably hard to translate properly...

Basically the story is that Sayaka's parents are out so she decides to take the opportunity to masturbate in her room using a pink rotor she stole from Mami. She's masturbating to a video of Kyousuke playing the violin and reveling in the sinfulness of it. On page 10 she starts counting herself down to orgasm but on page 11 Kyouko bursts into the room with some snacks she bought.

Sayaka frantically tries to deny what she's doing but it doesn't matter because Kyouko apparently knows nothing about sex and thinks Sayaka has been wounded in the crotch. Then when the pink rotor pops out of Sayaka and Sayaka says "I'm dying" (metaphorically), Kyouko thinks her soul gem has fallen out and she's literally dying, so she tries to "put pressure on the wound" with her hands and tells Sayaka to try to hold on... but it's too late, and Sayaka can't help but orgasm in her friend's hands.

Page 17 is some time later when Kyouko sees Sayaka "dying again" (oh no!).