One thing that you should keep in mind is that in your approach you should :
1 be a genuine friend. 2. dont let your agenda be recognized in other words dont come on too strong 3. Introcuce them to other people in a non religious setting, like school or just friendly people. The more you can hook them up with other friends the more they can see that there is other good people outside thier religion. It takes time, patience, perseverence.
Know their weaknesses in their doctrine.