from the Mystic tan website:
"Swimming in chlorinated water or salt water can fade the Mystic Tan results, cause possible streaking and will diminish the length of your tan. However, normal activities such as bathing, showering or physical activities will not affect the quality of the tan."
I think I've seen someoen down there with this streaky effect, it wasn't very attractive
I recommend good sunblock, like everyone else is. She will probably be much happier with the results.