Safetyfreak, I entirely agree with you. My H had left his OW in no doubt that he would be leaving me for her - they even looked at houses together and bigger cars which could accommodate our three DC and her three...
When I found out and he told her I knew she immediately told her husband and went to her parents to wait for my H to 'collect her'. Meanwhile he is busy minimising it to me...
Once she realised that the arrival of the knight on a white charger was going to be a bit delayed she went all out - really all out - to destroy me first, both personally and professionally (we worked in the same organisation) and then after that - after my H had really broken it off with her, which took a while in reality, whatever he said to my face to the contrary - she put the boot into him professionally.
The impact of her campaign against me and then him has been incalculable, both in terms of my overall professional and personal self-confidence but also his career. It was his fault, I hold him entirely responsible , but it's another aspect of the very profound consequences these affairs can sometimes lead to.
It's a lie that your DH has only been with her a month. They are far, far too involved for that to be the case. My H told me it was 'six weeks' but I later found out that it was many months. And before they became physically intimate there had been a slow burn over a couple of years as they too worked together.
Please remember that he has lied to you without compunction all the time you didn't know. He obviously did that very easily and very convincingly. This is the same man you are now relying on to tell you the unvarnished truth.... I thought I really knew my husband, also one of the 'good guys', inside and out. I was wrong. Please don't be me.