Washington - If you relate, there is good news. (Yes, there is good news.) You know about this.
Many times parents have no clue. A child will often keep the relationship secret, fearing the parents' judgment about the age difference and avoiding an awkward or heated conversation. If you know about the relationship and know the person, even better.
Forbidding a teen to do something is courting trouble and is the quickest way to invite sneakiness and lies. But trying to not upset a teen is like trying to not get wet when you jump in a pool: It's going to happen.
Is your connection so tenuous that a conversation will push her into total shutdown? If it is, I have no judgment. You won't be the first parent who feels disconnected from their teenage. And if this is the case, you still have a choice between abdicating responsibility (and saying nothing) and taking the full nuclear option of demanding they not see each other.
If your connection id strained, start hanging out with her and ask thoughtful questions and listen. Do whatever she likes to do (shop, surf the web, ride bikes, kick soccer balls), and ask her/ him what she/ he likes about the adult she or he is dating, what makes him/ her interesting, and what they have in common. Practice listening without offering too much in the way of critique or worry.
I'm not saying this is easy, but just practice. As you get better at being with her and listening, you may find a way from unease to some comfort in discussing your worries and thoughts. Our goal isn't to avoid upset or big feelings; those will happen on their own. Our goal is to open a real discussion.
If your relationship is pretty good, it is time to create and uphold reasonable boundaries. It is easy to let rational boundaries slip away if upsetting emotions feel too big. This scenario is an invitation to step into your role as a strong parent. That means voicing your concerns and having a conversation with your child about his or her life and your expectations.
If this is threatening to you, I strongly suggest you seek a good therapist and find your voice.
If your teen is spending time with an adult who has an apartment and a full-time job there are developmental milestones that he or she has had (graduating from high school) that your child may be missing out on if s/he is sitting at his place, watching TV and waiting for him or her to get home from work.
And call me jaded, but I have a very strong suspicion that if they are not already sexually active, they will be soon. There is only so much unattended time two teens can spend together before the inevitable takes place. Ensure your child sees a doctor however awkward it may be - as it will pale in comparison with the upset of an unplanned pregnancy or sexually transmitted infection.
It is your duty as a parent to face this with honesty and compassion.
My challenge to you is to avoid the extremes of this dating scenario. It isn't "stop seeing him" or "pretend nothing is happening." These are false positions that will lead to misery. Another challenge: Find your voice and use it.
Teens who don't have boundaries and who don't have parents who actively engage them feel out of control and lost, and will tend to make dangerous decisions. Your child needs you to help navigate this relationship. Get in there.