Telling kids "The Truth" - how much do you say??

[gemini3]

My husbands ex had an affair with his best "friend".  While he was deployed she moved into an apartment with the "friend", took the children, and filed for divorce.  The children were four and seven at the time. 

Fast forward to now, and the kids are nine and twelve.  The twelve year old has been asking my husband why he and her mom got divorced.  This has come up a few times in the past, and my husband has always dodged the question by saying that he and their mom couldn't be together because they didn't get along very well and argued a lot, and that this was the best thing.  Recently she's been asking more questions, and being more admant about a more concrete answer.

About six months ago my husband was driving her and some of her friends to Lazer Tag, and overheard his twelve year old say to her friend that her parents got divorced because her mom "ran off with" (ex-friend).  He was shocked, but pretended not to hear it.

My question is this - she obviously knows what happened - because of what she said, and because she was old enough to remember moving from the marital home into the friends apartment and seeing them together.  I think that she keeps asking because she wants to talk to my husband about it.  I dont' think a twelve year old knows what to think about things that like.  Should my husband talk to her about it?  I can see both sides.  I know it's a grown up thing that kids don't need to know about - but she already knows because she lived it.  How can she know how to deal without her parents guidance?

Any advice is greatly appreciated.

[ocean]

Is the friend still around?
The next time she bring it up he can say almost what you said..."I was in the army and deployed and mom decided she wanted to leave and live with XYZ. When I got home we got divorced" Then go with the conversation...