It only took one time for Beth
First of all I thought this was one of the best movies on domestic violence I have ever seen. John Ritter's performance in this movie was phenomenal.
I thought it was noteworthy that Beth (the girlfriend) was completely done with Paul the first and only time he beat her up. Of course her beating landed her in the hospital near death so it was no small thing. I think Paul was attracted to Beth because of her strength and independence. She owned her own business and lived alone when he came into her life. What he didn't expect though was that this strong independent woman would NEVER tolerate an abusive man. She not only severed the relationship immediately but she made it clear that the only way she wouldn't file charges against him was if he signed up for the Quit program and completed it. It turned out though that that was the best thing that could have ever happened to Paul. He was finally able to turn his life around. If it had been Judy (who of course was abused over and over again all throughout their marriage) she would not only have never filed charges against him but would have never insisted that he attend a group or therapy or any such treatment. She was too scared to do anything like that.
I guess to sum it up, although Beth was severely injured because of Paul, his affair with her ended up being the best thing that could have ever happened to him and Judy and the kids. He finally got the help he needed.