If you are pretty certain about this then you must act. This is like saying, "I have a friend who is using a little Heroin once in a while" but worse. You can't be worried about him/her getting mad or upset with you.
This is one drug that will consume someone quickly, very quickly! I would get some wise counsel, i.e. a local professional, and confront the person now.
You might want to talk with a treatment center about an intervention. They know how to approach this problem. Don't try to handle it all by yourself. It is never too early to act concerning Meth. You just might save a life.