Good question. Being "born again," means you have committed to the Lord to keep his commandments. A person is then born again by the spirit of the Lord. The person is then a spiritual son or daughter of God/Christ because he has spiritually begotten that person. He has given that particular person a new spiritual life, apart from the world, with all guilt, chains and stains of sin removed. Many profess to be born again but are deep in sin and judgemental of others. It's okay to judge righteous judgement though, this is scriptural and we are commanded to warn our neighbors as we have been warned. Don't take it too harsh, your friend is only concerned about others and their eternal well being.
There is safety in obeying all God's commands, this is a true born again Christian...
If your friend judges you again say to him have YOU repented of all your sins. All of us have not of course and there is always room for improvement, even if we don't believe in Christ or are religious in any way...
It's a tough path to walk in this world but it's actually a lot easier to not sin than to sin. Things run a lot smoother when a person is free from the affects and chains of sin. It's freeing, for lack of a better description...
Being born again means that particular person is keeping the commandments as best as they can...