Friar Lawrence was right in the sense of trying hard to be a mentor to these two young ones, but that's as far as I'll go. When two people seek marriage in the Catholic Church, unless there is something significantly wrong, the wish is usually granted. Could one make a case for something significantly wrong here? Yes, in regards to age and parental consent; however, not in regards to the love and compatibility of the two. It's a toss-up.
More importantly, Friar Lawrence seems to view this as more of an obsession as time goes on, producing a sort of snowball effect (similar to someone who has to continue lying to get out of his original lie). For goodness sake, giving a young teen a potion that will make her seem dead so she will be buried!?! This is not the action of a rational person. By this point Friar Lawrence is guilty of the same irrational passion and desperation as the two young lovers.