What will you do if something like this is happening around you? Why would a drug dealer want to learn Bitcoin if not for a safer haven for running the business?
If you believe that by teaching him how to use bitcoin, it will help him overcome his busniess and start a new life as a better person, then sure but if you fear that he might use it to improve his drug dealing business, I would advise against it.
I mean It won't be that good to just use bitcoin for a way of payment of selling drugs, you just think what's the best for you and for him i guess:)