Talk it out, and make a point of, in the future as well, promising that you'll talk things out with one another if you're upset. Be honest but not harsh, sympathetic but not a pushover, and respect one another. You'll have fights or disagreements, doubts and questions, tough times and good times. Nothing ultimately came of her talking to this person, she told you about it, it doesn't sound like anything that's too big of a deal. Needing to take some time or talk to a friend when you're upset is understandable and acceptable.
You might feel wary for a time, but that's understandable here, too. Like most folks have said--talk about it, about why she did it, why you might feel hurt, and so forth.