You have every right as a Believer to tell anyone the truth.
Read: Titus 2:11-12,15
11 For the grace of God has been revealed, bringing salvation to all people. 12 And we are instructed to turn from godless living and sinful pleasures. We should live in this evil world with wisdom, righteousness, and devotion to God,
15 You must teach these things and encourage the believers to do them. You have the authority to correct them when necessary, so don’t let anyone disregard what you say.