Forbidding it is an act of challenging her to push the envelope on what you will accept.Spend time getting to know him and see how they interact. Although it didnt work out my relationship lasted three years.
It may not be fun to think about, but she may need some information that you don't want her to have as a parent, but she needs as a young adult. My parents had a sit down talk with both of us when we first started dating and there expectations If he came over we had to stay in the living room. He can learn about your family and I think that creates a better open line of communication. My family didn't seem comfortable at first then they met him. We have now been together five years and married for a year and a half.The best you can do is talk to her, tell her your concerns, and be there for her if her decisions backfire. Talk with your daughter and set boundaries together.The more you try and control the more they'll push the boundaries. Get to know him and figure out their feelings for each other.Maybe go out on a lunch date or picnic, something nice that says, "I love you, so let's talk and spend some time together." She may not understand at her age that even a couple years can be drastic for teens in terms of mental development, maturity, experience, and what they want to spend their time doing.