Evidence—or questions?
The questions might be easier. The friar has a big heart. Your job would be to keep pressing at him to make him admit his feelings of guilt. In fact, that would probably be a lot easier, because in a literal sense, it would be hard to convict him of murder.
The place I'd start would be to pound away at the fact that the friar suggested Juliet take this potion (when she was underage and seeking counsel). Ask things like, "Did you or did you not know Juliet was only 13 years of age? Did you or did you not advise her to take an untested potion to that would allow her to appear dead to the world?" Then I'd probably suggest that if she was in a coma /trance for three days, she was probably not in her right mind when she actually did kill herself, and that, combined with her being underage, would be where I'd focus.