I reccomend reading "Freedom of Mind" by Steven Hassan, or at least look up some of the material on his website www.freedomofmind.com
As has been said, don't outwardly oppose them. Be interested and ask detailed questions to get them to think about things. For some questions to ask, check out: http://www.jwfacts.com/watchtower/questions-to-ask-a-jehovahs-witness.php
Another tack to take would be if you talk to them about another group that they would agree is a cult. The mormons are a good example. Explain how these other groups use mind control techniques to indoctrinate people, and your friend will likely begin to see that the JWs use the very same techniques. One technique is to get them to tell you themselves what would cause them to end involvement with the cult. This will require some research on your part (jwfacts.com is great for this) to see what damning evidence there is towards the cult. Steven Hassan, in one of his videos, talked about how he questioned a JW in this fashion and got them to admit that if a Governing Body member (the GB is a committee at the top of the pyrimid in the cult) left the organization because they realized that what they taught wasn't from the bible. This indeed has happened, and the man wrote a book about it.
Look up youtube videos created by ex-members and have them watch them. If they're already to the point where they're weary of looking at anything from ex-members, ask them if they'd stay at a hotel or eat at a restaurant that had thousands of negative reviews just because the proprieters of the restaurant/hotel say that the place is wonderful? How much more important is the decision on what religion to join? Shouldn't they have ALL possible information in order to make an informed decision?
Also, don't use the word cult with them, use "group" or "organization" as the word cult will set off alarm bells in their head that tells them that you're attacking them.
Lastly, the key to all of it is that you cannot stop showing love to your friend. The love that they'll recieve in the cult is conditional upon their acceptance of cult doctrine and behavior - your love is unconditional and they will see the difference. Get other friends/family involved and present a uninted front of concern (not opposition) to them. As others have said, direct opposition will get you nowhere. The people studying with them have probably already told them that satan may cause close friends or family to oppose their study - if you oppose you'll just be playing into their predictions. You might want to put out the word to their family on this as well. Any opposition will seem to confirm that the people studying with them are from god, and you are being controlled by satan, because they predicted it. Love and genuine concern and interest is the only way to go.