

Not much more you can do than explain to them clearly why you think it’s wrong from the perspective of both sides, whilst always reiterating they you will continue to love and support her. You need to explain why this man might be interested in her due to her age, and also that he may have enough experience in life to effectively lie and manipulate without her realising. At best you should get someone she trusts well to also explain this, preferably an adult outside of your household.
You can look into counselling, but only if she is willing.
I would not directly intervene until you feel it could become an unsafe situation. I would also not be seen as promoting the relationship either until you are also clear this person.
That’s somehow even worse than my original mental image. Great work science!