You could defend Friar Laurence by arguing that he only ever acted with the best of intentions. No one can seriously argue that the good friar was ever malicious in facilitating the relationship between the two young lovebirds. He could tell that they were in love and thought it only right and proper that, as both a responsible adult and as a man of the cloth, it was his duty to bring them together, so that's precisely what he did. The subsequent tragedy of Romeo and Juliet's deaths has nothing to do with him; their deaths arose out of a set of unforeseen circumstances over which neither Friar Laurence nor anyone else had any control.

One could go on to argue that the only reason why the friar was placed in this position was because Romeo and Juliet's parents effectively abrogated their responsibilities as parents. Instead of doing what was best for their children, they chose to continue pursuing a pointless, bloody feud that made it impossible for Romeo and Juliet to express their love for each other without resorting to the kind of subterfuge facilitated by Friar Laurence.

I agree that you'd have an uphill battle for defending him, but there are a few defenses you might postulate.

Probably the best defense would be love itself. Many perceive Romeo and Juilet as the "star-cross'd lovers" who have been unfairly parted. It might be argued that Friar Lawrence wants their paths to come together. He will do whatever it takes (even the ill-considered sleep potion) to unite the young lovers.

Another defense might be that of allowing young lovers to make their own decisions. (Not a good idea, but it might be said!) The friar has asked Romeo to consider all the ramifications (his forgotten love Rosalin, banishment, disinheritance) and sees that he will not listen, saying, "O, then I see that mad men have no ears" (3.3.61). He means that those in love will do what they want anyway, so he may as well help rather than bluster to deaf ears.

A third defense may be that the Friar acts as a male role model for Romeo. His father has failed him, the other adults he knows are unreliable, and even his best friend doesn't understand him. It is the friar who urges the crestfallen Romeo to "Stand, and you be a man./For Juliet's sake, for her sake, rise and stand" (3.3.86-88). When that doesn't work, he resorts to the age-old, "You're acting like a girl," taunt: "Hold thy desperate hand./Art thou a man? Thy form cries out thou art./Thy tears are womanish, thy wild acts denote/The unreasonable fury of a beast./Unseemly woman in a seeming man,/And ill-beseeming beast in seeming both."

So, that's what I'd go with: love, letting young people make their own choices after counsel, and being a male-role model. Whether or not this is enough to get him off for a murder charge, I dunno.

It is actually pretty easy to defend the Friar in a court case for a charge of murder. (Now, for other things, it might be harder.) Murder is usually a mix of the deed done (what the person actually did) and motive.

Let us start with the action: did the Friar actually kill anyone? Did he, actually and physically, do the deed?

The answer? No. He did not actually kill anyone, and so any charges of murder fall apart. (Accessory he might be guilty of, and perhaps other things.) He provided the potion, but Juliet took it. He helped with the plans, but didn't execute it, and so on.

Now, turn to motive. Is there any evidence that the Friar had "malice aforethought" regarding Romeo and /or Juliet? Is he known to dislike them, to benefit from their deaths, etc.? No, he is not. In fact, he's known to be on their side and trying to help them. In fact, you could lead him through testimony showing that a) Romeo came to him, b) he tried to make things work out, and c) he tried to get them to calm down.