Bear in mind, if she is scared shitless, and apologetic, it is not because she has any regards for your feelings, it is because she is worried for herself. She has gone into full "damage limitation mode".
If she did not know that you and your dh are divorcing over this, this knowledge is seriously going to rock her boat, as NOW your husband is free to pursue her full time, and she wont like that as he may up his stakes, and this way ruin HER marriage.
You should tell her husband, she is an insensitive callous bitch.
Also, dont forget to name her as the adulteress on your divorce papers.
She is pleading the cancer card, in an attempt to prevent you to talk to her husband.
Do tell him.
There are women like that everywhere.
An aquaintence (sp) of mine is divorcing. She has 4 children. Her husband had an affair with a mum in her oldest childs class (year 1)!! She wont uproot her child from school, so has to see OW every day. OW is happily continueing her marriage as her husband "knows what she is like".