Firstly, I sorry to hear about your situation, MrsO. It must be a difficult time for you.
but I agree with cazzz, as a Bloke in my 30's I would be absolutely horrified and would be worried sick if I ever got caught out by a under age girl lying to me, But due to the fact I know teenagers lie about their age I would be very very cautious of anyone who told me they were 18. (I'd also think even 18 is too young for me, but I'm at the upper end of my 30's)
I would agree that a quiet, off the record, word with the bloke would be in order, I'd also hope the police would have a stern word with your daughter about putting herself and him in this situation by being deceitful.