<human>: Can you give me the gist of the text in a nutshell?

So my best friend of 10 years who we'll call (X) who I am actually joining the Army with in this next week has started cheating with my other friend of a few years, (Y), girlfriend (Z). Y and Z have been happily together for the better half of 2 years now and we all hang out on a regular basis. 

Now X and myself share just about anything and everything that goes on, as guy best friends do, so naturally 2 weeks ago he informed me that after a group outing to the movies he and Z starting texting. She proclaimed that she had to go to her apartment alone because her boyfriend Y had to go home to study. Well one thing lead to another (he skipped the small details) and he ended up driving out to her apartment and they had sex. He showed me proof the following days with the morning after texts from her.

 I was so caught by surprise; I didn't think my friend was capable of something like this and especially with someone he's known for so long. Over this past two weeks they have had a few more encounters very similar to the first one. What bothers me the most is that we all still get hangout on a regular basis, and seeing them and  knowing the situation is killing me. Y is a great friend of mine and I don't feel right keeping it from him. But I know that if I confess what I have learned not only will their relationship end, me and my best friends, X,  will diminish. The shitty part is that we are joining the Army together and will go to basic training together and have a strong chance of living with each other in advanced training. I  can sort of see the justification because we are leaving in a week but we are coming home for 2 weeks at Christmas time and I fear they are going to continue. I'm just so torn on what should I do. 

Also, I know that both X and Z are equally at fault and my respect for them is lost. 
Thanks for any advice.
<bot>: Best friend (joining Army with) is cheating with other friends girlfriend and I don't know if I should tell the boyfriend.