As you say, she is of age. She could even get married if she wants.
It is possible that this 28-year-old is not out for a quick hook-up. You might find that he is as good a match for your daughter as any other guy. By all means meet him (and believe me, it is a good sign that she would like you to). Your restrictive approach now is only going to have the effect of convincing her that they are Romeo and Juliet.
She is 18. Inlet her live her life before you push her away and lose her forever
I think She should find someone mature and who truly loves her regardless of the age. I'm 23 and I usually have to talk to girls my age or older just to have normal and casual conversation. I think she would definitely be better off with someone that's not only mature but actually loves her and wants to take care of her.
I know this is an older post, but just wanted to chime in with something. I was 19 when I met my husband..he was 31. My mom didn't like the age difference, but she eventually gave him a chance. She saw what I saw; a loving, caring, compassionate, hard working, stable man. We have been married for over 10 years now, he's my best friend and greatest joy. Don't assume this man is after a booty call...he may be an awesome man! Get to know him, see how he really is, give him an actual chance without judgement. He may end up being the best thing that ever happens to your daughter :)