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.