If you are already having doubts about this friend, it could be the reason for having this dream. I've found that if something bothers you or if you think about it enough it can come up in your dreams. If could be your subconscious trying to help you work through the issue.
If you feel the dream is prophetic, look for signs in your day to day life that could relate to the dream. Do you notice anything that could relate to your dream? Do you see any similarities?
The evidence needed to prove that a dream was prophetic really depends upon the dreamer and who is willing to believe them. What makes a dream prophetic really depends upon belief, belief of the dreamer and the belief of the audience (who the dreamer tells).
The best way to know if a dream is prophetic is to have a better understanding of your dreams and the meanings of elements in your dreams. Yet since dreams can range in occurrence there isnt a catch-all for determining if a dream is prophetic in absolute terms. A person may not recall any of their dreams and then all of a sudden is able to recall one dream that has bearing on events in their life or the world around them. Another person may never seem to have a prophetic dream at all. The value a person gives to their dreams also has bearing on their interpretation and importance. From what I have gathered from personal research is that prophetic dreams tend to be vivid, detailed (specific items stick out), emotional and may cause a sense of urgency or concern in the dreamer depending on the kind of prophetic dream. Recording your dreams even just a word or phrase can help a great deal in remembering future dreams. The more you pay attention to your dreams the more you will learn about them and yourself.
Please do not forget that every dream might have a meaning or something to "tell" you, it all depends on what you think and have in your mind. Please notice that not all what happens in a dream has a direct meaning / interpretation, sometimes you have to look at dream differently and see if it is something in your Subconscious which causes the dream. Feelings and emotions, worries and fears are most likely the stuff that cause your dream, try to think of what you have in your mind.