I had talked with a friend who wanted to tell him as he went through the same thing.  His wife was concerned because of reports that he husband might be violent.  So - I told my friend that IF his wife was having issues with it to PLEASE not do anything as it was much important for him to listen to his wife. 

Well --- I then decided after a lot of thought that I WOULDN'T tell him - until I knew for sure what my motives were.
I felt fine with my decision.  I just waited to hear from my friend so I could tell him that I didn't want him to  make the call.
In the meantime - I went to a client's house & on my way back I got a call from my stbxh telling me that someone had called the husband & that the husband had phoned him - quite irate!  I was honestly quite stunned as I thought - wow --- someone else wanted to tell him too.  I didn't think there was any way my friend made the contact.  So I told my stbxh that I had no idea who did it.  Then I found out that it WAS in fact my friend after talking with him.      I was stunned (& a little upset as he didn't call me 1st).  He felt that he had the "right" to call the guy as he felt it was the best thing to do. 

My stbxh believed me @ 1st that I didn't do it.  The fact that I am 1000 miles away helped a bit.  
Then the girlfriend got SO mad at me - she broke up with my stbx.  She blamed me for ruining her life.  I have to say I find it really funny how she took NO responsibility for why her life could be a mess!  She called my ex and told him to make sure to thank me for her.  She was really furious - which for some strange reason did NOT make me feel bad at all.  Seemed like a nice little bonus!  : )

This of course pushed my stbx over the edge and he lost it.  He told me that he never wanted to talk with me again.  While that sounded kind of nice I had to sadly break it to him that he HAD to communicate with me as he was having our son live with while I travel around for the next 8 months.  He pouted for a few days.  He EVEN had the nerve to write  me that I just had NO idea how upset & SAD he was.   Boo hoo!  Are you kidding me?  I wrote back that I kind of understood the whole upset and sad angle - as his girlfriend of 6 months broke up with him while my husband of 26 years did the same thing.  

He decided to talk to me again.  Oh joy.  I told him that I was MORE than happy to give him all the space he desired.  I also said that he MAY have a bit of displaced anger.  After all the love of his life - gave him up right away.  The letter did NOT mention his name.  She also was finally "free" and what did she do?  Run to him?  Nope --- she runs the other way.  This is after he leaves his family  & gives up his oldest son for her.  Funny how her excuses for not leaving her husband meant nothing went push came to shove.  This was her "out" === but it ended up being her out from the relationship with my husband.  I thought it was sad that he gave up so much for her and she dropped him as soon as she was caught.  She kept saying that she couldn't tell her husband for a number of reasons.  Suddenly all of those obstacles were removed & she dumps him.
 
So here it is a week later --- his "love" is still not with him.  He is talking to me when needed.  I am glad that the husband found out.  I am glad I wasn't directly responsible for this info.  
I do think he had the right to know about it.  I am just glad that I am able to move on a bit more now.
Thanks to all of you for your help and suggestions.