Hi, jp. Stevia is a dietary supplement. Dietary supplements are not regulated by the FDA; therefore, the amount of the item in the bottle may or may not match the label on the bottle says it contains. Your friend could end up taking much more than intended.
Here is another link you may be interested in: Diabetes - Life Lines
I would suggest that your friend ask his doctor or a diabetes health educator whether stevia is okay for him. Even all-natural items can interact with medications or other dietary supplements and have negative effects.