Well, I'm sure if he would've told him after he married her Lord Capulet would just have to deal it. But before how would he act?
Judging from Lord Capulet's response to Juliet's refusal to marry Paris, I think it is fair to say that his reaction would have been extreme, if not violent.
Lord Capulet threatened to throw Juliet into the street and disown her if she didn't marry Paris. I can only imagine his response would have been similar had he learned of Romeo and Juliet's intentions to marry one another.
In my opinion, there is no doubt about what would have happened. Romeo and Juliet would never have gotten married.
If the Friar had told Lord Capulet, he would have made sure that Juliet never got to see Romeo again. I know that Romeo sneaked in to see Juliet, but if Capulet had known that could happen, he would have had people watching to make sure it did not happen.
If he had had to, he might have even sent a message to Lord Montague telling him what was going on. He would have known that Romeo's dad would not have liked the romance anymore than he himself did.
So if Friar Lawrence had told, Romeo and Juliet would not have married and would not have died.