Thank you for bringing this forward and for caring enough to seek wisdom before acting. What you are describing is serious, and your desire to handle it wisely is right. Scripture says wisdom is the principal thing, therefore get wisdom and with all your getting get understanding. The fear of the Lord is the beginning of wisdom, and to depart from evil is understanding. That means wisdom does not look away from evil, it hates evil and moves away from it and helps others move away from it as well.
There is a wisdom that comes from above and a wisdom that is earthly. Earthly wisdom might tell you to stay quiet to keep the peace, to avoid making waves, or to protect yourself by ignoring what you have seen. That kind of wisdom leads to envy and strife and leaves people unprotected, and God says it is not true wisdom at all. The wisdom that comes from above is first pure, then peaceable, gentle, and full of mercy and good fruits. It leads to purity rather than sensuality, to righteousness and clear judgment. When you see an adult acting inappropriately toward teenagers, true wisdom will always move toward purity and protection, not toward covering over sensual behavior.
Wisdom also dwells with prudence. It is a defense and it gives life to those who have it, and it preserves and keeps those who love it. Wisdom cries out in the open, in the streets and at the gates, inviting everyone to come and learn. In other words, wisdom is not hidden or silent when harm is happening. It speaks up in the right place and in the right way.
You mentioned that this man talks about bringing a gun to work and seems unstable, and that you do not want to get anyone into trouble. That concern shows prudence, and prudence is part of wisdom. It is wise not to confront an unstable and potentially armed person alone. It is also wise not to stay silent while younger women may be at risk. Loving wisdom means you will seek to protect the vulnerable while also acting carefully and lawfully.
Ask God for wisdom as Solomon did. He asked not for riches or position but for an understanding heart to judge rightly, and God was pleased to give it. In Christ are hidden all the treasures of wisdom and knowledge, and God promises to give wisdom generously to those who ask in faith. Pray first for that wisdom, and then act on it.
In practical terms, wisdom would lead you to do these things:
Document clearly what you have seen and heard, with dates and details, without exaggeration. Keep that record private and accurate.
Do not try to handle this alone or confront him directly about the gun or the behavior. Bring your concern to management or human resources promptly, and if your workplace has a formal reporting process, use it. If possible, bring a trusted witness or put your concern in writing so there is a clear record. Because teenagers are involved and a firearm has been mentioned, wisdom also means being prepared to involve proper authorities outside the workplace if management does not act quickly to protect those young women. Protecting minors from inappropriate behavior is not getting someone into trouble unjustly, it is doing what is right and hating what God hates.
Seek counsel from a mature, godly person you trust who can walk with you through the process and help you stay steady and truthful.
Continue to pray for the young women, for management to act with integrity and courage, and for this man to be stopped from harming others and to be brought to a place where he faces his own need for repentance and help.
I am praying with you that God will give you a clear, understanding heart, that He will guide your words so they are truthful and righteous with nothing perverse in them, and that He will use you to bring protection and purity into this situation. May He preserve you and keep you as you walk in the fear of the Lord, and may He establish your steps with the wisdom that comes from above, which is better than rubies and better than any earthly advantage.
Father, in the name of Jesus, I ask for wisdom for this dear one who is carrying this burden. Grant an understanding heart to know what to say and when to say it, and to act with both courage and prudence. Expose what is evil, protect those who are vulnerable, and let truth and purity prevail in this workplace. Give safety to all involved, guide those in authority to act justly, and draw each heart to the fear of the Lord which is the beginning of wisdom. Amen.