Hi, I understand how this can be bothersome, but it seems that right now you both are seeing each other right now and are not in a serious relationship just yet. I do commend her for being honest with you despite you both not having a commitment at the present time. It does not seem as if it was her fault. I do not know the details of who this person is and if she knows him or not, but it is good that she was honest. She also did not allow it to go any further. She should just be clear with this person in order for it to not happen again because whether or not she is with someone people need to respect her as an individual. It would be important for her to pass this maturity to ohers, so they would know that is not acceptable unless it as a stranger which would have not been acceptable anyway. From what you said it seems like it was a random person. In that case I would not hold responsible. I would thank her for being honest and continue the relationship as you were. If you see more suspicions or strange behaviors that would be concerning. As just for this I would not worry. I wish you both well and please let me know if I can be of further help.
I would like to help you. Please let me know if I can help your further and why you were unhappy.
I was being honest with you and gave my honest opinion. Not sure what you are looking for, but if you can give me feedback then I can understand better. Thanks