I firmly believe that if the circumstances are there, even the strongest partnerships are vulnerable to this sort of situation.
Of course no-one means to hurt anyone, but that doesn't mean no-one gets hurt. if that were the case, anytime a car ran over a pedestrian, the pedestrian would be unharmed because the motorist is a stranger who meant them no harm.
Sadly, life is not that simple.
I know from counselling experience that people who are emotionally vulnerable fixate very easily on people who offer them support - they imagine a seriously strong sexual attraction, however unlikely and inappropriate it may be, and if the other individual acts on it, you have an affair up and running.
The husband blaming your workmate is again classic displacement - if he can make her the guilty party, it stops him from having to assume that role with al the guilt and pain attached.
It sounds like your friend will separate from her husband, and I aplaud you for supporting her, even though it is evoking some very unpleasant memories for you.
As you know, time lends a sense of distance, and it is possible to assimilate these exerpeinces into your life - which is not the same as 'getting over it'. Hopefully, with your personal experience to draw on, you can both move forward together.