I know of a few people who are born again believers but who have gone back under the law, even though they are not Jewish. They practice all the laws and some men have become circumcised. The Holy Spirit has shown me that is not required of us, as has the bible. How does one show these people they are being foolish?
In my experience, trying to show people that they’re being foolish only makes them more determined to prove that they’re right. If the people you refer to know they’re saved by grace through faith and are truly born again, then their behavior can’t do them any harm. If they’re not, then their behavior can’t do them any good. We’re saved because of what we believe, not because of how we behave (John 3:16). The problem is, you and I can not tell which is true in their case. Only God knows their heart.