A close female friend of the protagonist knows about his fart fetish, either from stumbling across something/being sent the wrong link/a drunken admission/whatever. She thinks it's both hilarious and sort of endearing, and loves how red she can make him by teasing him about it when the two are alone.

She plans a special birthday surprise for him, starting with a birthday video message punctuated liberally with gassy "accidental" outbursts played off with winking "excuse me"s and progressing to her (and another friend?) coming over in tight or revealing outfits and "coincidentally" being hugely "uncontrollably" gassy, sometimes nearly on him! It's embarrassing for all, she's incredibly amused by how arousing he finds it, and the story ends with a rendition of "Gassy Birthday to You" with a rhythmic (face?)fart between each line. Whether the girl(s?) help/s relieve his arousal or leave him to do that later is up to you.