"Flip" is what one does in the darkroom (or on the computer); "Flopped" is the description of the resulting image. "Flipped" is an acknowleged American alternative spelling. See the dictionary entry for "Lens versus Lense" for further information.
In labs I worked in, when that result was desired, the work order never had anything but "FLOP" scrawled over it. "Flip" may be correct according to some dictionary or another, but I never encountered it in practice.