I have used Grandma's Secret Spot Remover. Bought mine from Amazon:
http://www.amazon.com/Grandmas-Secre...dp/B0021JD2S0/
I haven't used it on the type of rust spot you describe, but I think it would be safe to try it. So far it has worked on three different stains for me in different garments and fabric types.
Check the Tide stick for ingredients, as any kind of bleach will weaken the fibers (even if it takes out the stain). I'm not sure it would work on a rust stain anyway as this is a mineral stain, not an organic one.
Edit: I found this article online that says that salt, lemon juice and sunlight works pretty well on rust spots:
http://laundry.about.com/od/stainrem...removerust.htm
The only thing is, if she leaves the block in the sun, she is going to want to cover the Dresden plate portion of it with something that will let no light reach the fabric (a book would be good). Sunlight fades fabric dyes fast!