Stance on divorce for affair, with kids?

Subsequent to the extra-marital affairs thread, and a real-life situation posted about in a marriage forum I follow. What are y'all's thoughts?

The woman has a child with her husband, and he has had some borderline infidelity issues in the past (online flirting bordering on cybering) and also is jealous of her, snoops her business and her exes, and makes cruel and hurtful remarks out of anger or that he says are jokes. Now he is looking for 'arrangements' through Craigslist's sex postings and emailing people, telling them his wife is out of town, etc, but says 'nothing has happened' and is mad at her for haing read his email, which he left up on the PC! And this is just a brief summary of his offenses.

She wants her son to have a father figure, but I and other commenters seem to agree: her son would do better with an amicably split home than growing up seeing his father treat his mother terribly, teaching him that it's ok to do to -his- partners when he grows up, or that if someone abuses -him- he should put up with it quietly. But I am perhaps biased, I was raised with a single mother who had a tendency towards getting into abusive relationships like this, my immediate reaction will always be, Get out and get out now!

What would you do in this situation? How bad do you think the spouse has to be, before leaving them is better for the children than staying would be? I am very curious as to what such a diverse and well-reasoned group as this has to say!