My sister has a LEGO minifigure attached to a key chain, but she wants to remove it from the key chain, how can she do this?
There is also a third method, using a cheap and readily available soldering-iron. The benefit is that it is a much faster and more reliable method than trying to pull the pin out by sheer force, which may or may not work depending on how strong you are. With a soldering-iron the pin just pulls out within a couple of seconds, no force required. See tutorial and video here: http://thebrickblogger.com/2011/01/lego-keychain-removal/
Most of the key chain minifigures are standard minifigs that have a large pin with a ring on top running through the top of their head into their torso.
There are two main ways to deal with it:
Cut it off.
This is straightforward. You can use a large pair of linemen's pliers or something similar to make the cut. You could also use a hacksaw of some kind. The main disadvantage of this method is that the head will still be permanently attached.
Pull it out.
Pulling it out is a little trickier. One method is to put something through the ring (such as a paperclip) and then pull hard to try to remove the pin. The pin is ribbed, so this could be difficult. This can be made easier by heating the pin first (perhaps with a soldering iron as TheBrickBlogger suggests) in order to slightly melt the plastic surrounding it.