Afraid there is really no magic cure. Did the trainer cut it or something? If so, wow, what a huge no-no. I always thought it was common courtesy to leave the manes and tails alone when you have someone else's horse unless given instruction to trim/pull ect.
Anyhow, I've heard of people using duct tape to keep the tail wrap more secure. Ha. Really though, you'll need to check it often and re wrap if you want it to remain, because they will find a way to relieve themselves of it eventually.
You could also try, as an alternative to the duct tape or with it, using vetrap (or any cheaper knock-off) to wrap the dock and the wrap.
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers