now-ex = ex-husband; now, at the time of talking, he is the ex-husband
then not-so-ex = ex-girlfriend then at the time of the wedding.
The woman made up the phrase not-so-ex to indicate the kind of relationship her then to be husband was having with his ex-girlfriend. In light of the fact the man slept with her, she refers to the ex-boyfriend as the not-so-ex.
This car is good. = a good car
That car is not so good. = a not-so-good car