Should I or shouldnt I?

This morning, as I was loitering at the reception desk, I saw my mentee asked the office boy to send a letter to Company A. Company A is of a very important client so I was curious about the letter. When I saw that the paper in the letter (thru the envelope window) is not of the paper we usually used for printing, I sensed that something is not right. So, I secretly took the envelope to have a look at it after a quick consultation with another colleague. My intuition was right as I found the letter to be completely inappropriate to be sent out at all. My mistake began here as I felt that this matter should be brought to my senior's attention immediately. I brought the letter secretly to my senior and let her have a look at it which lead to the order that I should filter thru all the outgoing document from my mentee. After settling into my cubicle, I was startled with the voice of my mentee asking for the client of Company A. As I watched in horror, she was indeed talking on the phone with the client. Panic took over me as I went immediately to my senior to tell her of this situation. She immediately went out to remedy the situation. From that moment on, I have another new responsibility. I now have to make phone calls for my mentee as well. My senior continued to lecture my mentee on the proper way to treat clients and how to write to clients after she finished apologizing to the client. I only realised that I might have made a mistake doing all the above when another colleague asked me why didnt I just point out the mistake to my mentee when I first read the letter. Did I really do wrong? Have I accidentally stab a knife to the back of my mentee? Have I accidentally created a image of a whistleblower to my senior? I felt that I can't point out the mistake to my mentee because I am not good at arguing or making my point. But if I have pointed out her mistake openly, I think, she will feel that i am stepping over the boundary for spying on her though mentor is supposed to look out for the mentee. Argh! Have I become a bad person by doing that?Somehow, I felt like defending myself...According to someone's assessment of my mentee's character, she is a person who like to destroy evidence of her stupidity and error and refuse to admit her mistakes. She likes to do things her own way and not listening to other people (me included). If i have not let my senior know about this matter, it might happen again and I cannot be there all the time to stop it from happening. I dont want to be bombed for not knowing what my mentee is doing. This is frustrating...I am in a dilemma...should I be feeling guilty or not?