oh, I find this question very interesting!
The difficult part is that it is basically a yes or no question, which I've always found the Tarot reluctant to answer.
I'd try to answer it with a very simple spread, maybe start by pulling out the Sun card both to represent shining light on the situation, and also to represent the child.
Then pull two cards, the one on your left would indicate the likelihood that he is not the father, the one on the right would indicate the likelihood that he is the father.
I think your friend's reaction to the paternity test raises all kinds of interesting questions whose answers might be more helpful in the long run.