It might depend some on whether they're actively going to be a temptation to return to previous ways. If you have to choose between a friend and Jesus, you know who you have to choose.
On the other hand, I do really believe that God wants us to love others, and I believe that friendship is a good thing. You have the potential to be a force for good in your friends' lives, both just by being a loving and supportive friend (the sort of friend that Christ was), and by being an example of the Christian life.
I would personally say that unless you feel like your own soul is in danger, that you should keep an old friendship alive. But of course I don't know the specifics here.