Hi.
First of all, challenge the person. Let him or her prove than Jesus said that is was only to be prayed in private. It can’t be done because Jesus doesn’t say that about the Lord’s prayer in Scripture.
Now He did say, “But when you pray, go into your room and shut the door and pray to your Father who is in secret; and your Father who sees in secret will reward you.” (Mt1:6) The context here is that He is warning them against acting as hypocrites. But He is not saying that they should never pray together.
He also says, “Again I say to you, if two of you agree on earth about anything they ask, it will be done for them by my Father in heaven. For where two or three are gathered in my name, there am I in the midst of them.” (Mt. !8:19)
Fr. Vincent Serpa, O.P.