As always, a little bit of critical thought and intellectual honesty will go a very long way when analysing such works as the Urantia book.
The work is meant to be a near omniscient account of all spiritual reality, of the structure of the cosmos, of human society, culture, and history, and many other things which all amount to being an incredibly vast set of assumptions. What validates such assumptions, beyond a person merely having confidence in the original claims of channeling which the Urantia Foundation puts forth? What sort of criteria could we establish, which would separate true omniscience from unfounded speculation written by educated, human minds? How is the Urantia book analysed by devotees of it? Is scepticism ever a viable form of analysis?
But of course, the best sort of person who can argue against a very complicated set of ideas, philosophies, and religious constructs, is a person who has spent a great deal of time intellectually sympathising with such positions. If you do not have time or will for this, or if you are not already a former Urantia book reader who found sufficient reason to abandon his beliefs, then perhaps you are best left simply arguing your own religious positions, if you so desire.
The claims of Christian orthodoxy and tradition undermine the validity of many things, including this Urantia book. Since your friends seem to be more and more walking away from beliefs you view as being inherently true, and since you seem to want them to return to your own ideals and perceptions of spiritual truth, then I would say that you should simply offer to them the same apologetics as you would to anyone else who is not a devoted Catholic.