I have a friend who slept with a married man at one time. It made me begin to wonder if she had commited Adultery when she did this. She is a single woman. Thus the question begs do single people commit adultery if they sleep with a married person? Is there a term for the single person who does this or is it a mortal sin of fornication? Thank you for your responses.
From the Old Catholic Encyclopedia
Adultery is defined as carnal connection between a married person and one unmarried, or between a married person and the spouse of another. It is seen to differ from fornication in that it supposes the marriage of one or both of the agents.
So, she comitted adultery.
God Bless
Just a question, if I may…Why need one be so definitive when it comes to a grave and disordered act ? Doesn’t the act define itself??..teachccd 
Welcome to the forum. Glad to have you aboard!! 
This is an interesting and valid question. The answer, of course, is yes, as others have already said.
The reason some people get so confused is that in the laws of most U.S. states, only the married person has committed the CRIME of adultery. (Yes adultery is still criminal in some states, and has civil implicatons in all states.) Both have committed the sin of adultery. All though all sin must be judged in its own context, and by God not us, in my judgment adultery is generally more serious than fornication because of the damage it causes to a marriage. Adultery is even more serious for the married guy because he is breaking his oath before God (assuming a religous wedding) and his duties to his wife.
So I am just adding this to point out that 1) your question is very legitimate, many don’t understand this and 2) your friend should discuss her sin with a confessor, who can help her deal with it in context and give her the spiritual guidance she needs.
You are a good friend to care about the state of her soul. I hope you can help her get the help she needs.
THE SIXTH COMMANDMENT
You shall not commit adultery.113 You have heard that it was said, “You shall not commit adultery.” But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart.114
2353 *Fornication *is carnal union between an unmarried man and an unmarried woman. It is gravely contrary to the dignity of persons and of human sexuality which is naturally ordered to the good of spouses and the generation and education of children. Moreover, it is a grave scandal when there is corruption of the young.
2380 *Adultery *refers to marital infidelity. When two partners, of whom at least one is married to another party, have sexual relations - even transient ones - they commit adultery. Christ condemns even adultery of mere desire.171 The sixth commandment and the New Testament forbid adultery absolutely.172 The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry.173
Even if a man looks lustfully at a woman, he has committed adultery in his heart. This can go both ways. Your friend who slept with a married man did commit adultery. Since by defnition, adultery is any sexual act that a spouse has with someone whom they are not married to.
While the question at hand is legitimate, the emphasis should be on the gravity of the act and not as much on its definition. Fornication and adultery equally constitute grave matter and with the proper knowledge and consent will lead to mortal sin. So, do single people commit adultery? The answer is that single people engaging in sexual encounters commit sin, and with the three conditions being met will result in mortal sin. If one should die in this unrepented state they will go to hell.
Seek prayer and place your heart and mind in God. If one understands chastity this question is actually not necessary…God Bless…teachccd
I don’t know what you mean by “the act defin[ing] itself”. Acts don’t define themselves in the sense that we automatically know what we know what every act is. We give them definitions in order that when we we talk about them everyone knows that we are talking about the same thing. Surely you also define terms when you teach CCD, do you not?
Yes, of course. I was alluding to the fact that the definition in this case does not reomve the culpabilty of the sin. Whether it is defined as fornication or adultry it is ultimately defined as grave matter. That is what I was referring to. That’s what I meant when I said that the act defines itself. Sex before marriage is offending to God and disrepectful to your partner and self. But if one must have the proper term for this offense, then so be it…teachccd
If one presupposes that all premarital sex is wrong then there is an obvious implication that man (non-gender specific) is meant to have one sexual partner, namely your spouse then there is an arguement that could be made that the person who was un-married has committed adultery to their future spouse, assuming one follows the belief that we are meant to go forth and procreate.
Its a bit of a stretch TBH but is interesting and not something I have thought of before. . . . thanks for the dilema!