I just have to add my support to Black Sheep's statement. It is important to completely understand the shades of meaning. The person you study with is being "technically" honest, but extremely dishonest about the spirit and practice of association.
You CAN have "worldly" associates. There is no danger of DF/DA. But there are social sanctions. If a young person has non-JW friends, then parents of other JW young people will deem HER a bad associate also and not allow their children to hang out with her. If you have a party and invite JW's--you won't be able to comfortably invite non-JW's. If you do, JW's might then decide that you too are a bad associate. Bad associates are not only non-JW's, but JW's that don't follow "suggestions" on association. If you want to get married and you would like your best non-JW friend to stand with you--forget it. Not in the Kingdom Hall anyway. And you won't go to their weddings if they are in a church.
But there is also something else to consider. There will be a NATURAL pulling away on your part and on the part of your non-JW friends. Think about it. If you immerse yourself in the JW culture, what will you have in common with them? They will want to talk about Christmas shopping. You'll be silent. They will want to talk about dating--what will you say? Will you celebrate their birthdays? Will you bless them when they sneeze? What happens when they offer an impromptu toast? This has happened to me a number of times. Imagine sitting at a table with a few friends and on offering a toast and you are the only one not participating. Go to a sports event with them and see how you feel when you don't take part in the national anthem. Can you go out with them to a bar or a movie? Can you even talk about movies or books or music? If you totally immerse yourself all you will have to discuss are meetings, bible study, service and other witnesses. They really won't want to hear about that. And if you don't immerse yourself you will be seen as weak in the congregation.
When I became JW I pulled away from my friends because we had nothing to talk about. It just happened naturally. There was an article once that said that Israelites used to be separated from the nations by their distinctive dress (blue thread, fringe, no mixing of fabrics) and customs. The article said that today JW's are separated by their behavior. And that was a very good thing.
So yes, you CAN have friends that are not JW in theory. In practice it is sticky and negative. The pressure will never EVER stop. If you make any mistakes they will immediately analyze your associates. And trust me, if you are having fun with a friend and it's meeting night, you really won't want to leave them to go sit in on a meeting that is boring, uncomfortable, and leaves you feeling guilty for having been with a worldly friend minutes before showing up at the hall. Also, you will be expected to witness to these friends. Nothing puts a wedge between friends like prosylitizing.
Think about these things. Once you are baptized you can never go back. I have lost 20 years worth of friendships that I worked hard on. Now they are dust. And there is no changing that.
NC