if you want it removable, why not put the magnet in a simple strip-belt under her clothing? if its t-shirt cotton it will be pretty thin but strong enough to hold, and you can either tie or stitch it on, and when you decide not to use it any more just cut it off her body. make sure the polarity you need is against the thinnest layer of fabric if you can, the more layers between the magnets can effect the strength depending on the magnet type.
kinda like this cruddy 30second mspaint doodle:

stick magnet in middle, fold over fabric, and stitch closed close to the magnet to keep it from wandering off. then just do whatever to the ends. heh.