Błędnie: Her girlfriends are simply jealous. They won't be happy when they find her involved in another flirt again. Poprawnie: Her girlfriends are simply jealous. They won't be happy when they find her flirting again.
"Flirt" to 'flirtować', jako rzeczownik oznacza 'flirciarza', 'kokietkę'. 'Flirt' to po angielsku "flirtation".