Fair-weather Flirt

A friend or acquaintance who flirts with others only when they do not have a more desirable romantic interest:

Sally and I have been hitting it off lately via text, even exchanging some salacious images. I thought for sure we were going down a path of romantic exploration, but now that her ex-boyfriend, Frank, is back in her life, she turned out to be a fair-weather flirt and won’t even reply to me.