The only issue is that Friar John is going to Mantua because of Romeo’s actions. If Romeo thought about the consequences of killing Tybalt he would’ve never been there in the first place. Some may even say that the plague that Friar John got stuck in could’ve been Mercutio's curse he laid on the families but I digress. Others will say that Romeo and Juliet would’ve never have been a tragedy if Tybalt never fought and killed Mercutio and yet again I can see where you would think that. The problem with that statement is why Tybalt fought. It was because Romeo crashed the party on the first day and Tybalt noticed then he stated “This, by his voice, should be a Montague.- Fetch me my rapier, boy.” (1.5.60-65). The key word in that sentence is Montague. Tybalt would’ve never said anything if he was a Capulet or even if he didn’t have a side. It all ties in with the family feud