are you sure your friend didn't say if you stick a child in the freezer it will turn purple and not if you put your plants in there??
seriously though, what your friend is talking about is its a pretty well known fact that on some strains, colder temps will turn them purple.. this is usually done with the weather in the fall on outdoor grows in a more northern climate, and not indoors by sticking plants in the freezer, as i'm sure that would end up killing them instead of turning them purple..