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.