The girl who Billie has called Amanda (the "one that got away" who Good Riddance, Whatsername, She, Sassafras Roots, Amanda and a bunch of others are inspired by) is the girl who he's said he was with for about a year while living downstairs from her in the "Longview" house, him writing Dookie and her at college, who then went off to study in Ecuador when they split up. Is she the girl whose name you've heard is different?