I would talk to her about your concerns, but not in a OMG!! type of way. Ask her why she hid him from you. She was smart enough not to meet him alone, but she shouldn't have kept him from you overall. That wasn't so smart.
You can meet nice people online and weird people online. My BIL and sister met online. Two of my best friends (who are now married) met in a role playing chat room. So I'd talk to her about why he's coming, etc. How does she feel about him? What does she know about him?
If you do not meet him at your house, find somewhere else to meet. It would be completely acceptable to say no, but you'll meet him for dinner. Make sure he doesn't expect to stay with you. And see how it goes. Thank your DD for being honest with you now. Try to look at it as similar to meeting someone by chance that moved after the initial meeting. My friend flew out from Utah to meet her now DH.
And, to be honest, I met a guy in a bar, chatted so briefly, and knew less about him than some of the people I have met from online friendships. Yeah, people can lie. They can lie to your face, too.
So meet the guy. See how it goes. And tell DD you are just concerned that she felt she had to keep it from you and that she met him without telling you - just in case.
Fact is, she's 18 now and it's a weird place to be as a parent.