As I posted earlier, the reason I think this letter to Dear Abby was a fake is that the teenage girl herself would know her father really may kill her, and wouldn't be taking advice from a friend's mother on this. It is possible, as you wrote, that the mother who (supposedly) wrote the letter would not realize how dangerous her daughter's friend's father is, but, again, the girl herself would know that. Any teenager who comes from a household where one or both parents beats the kids up would visit more normal households and see the difference and know that the more normal people would never understand what chaotic households are like. Teenagers who come from the types of families Abby is ostensibly concerned with would ne unlikely to go to court to get abortions, anyway; they would runaway. The girls who would be organized enough to go to court to get around notifying their parents are most likely to be girls from solid families, girls who do not want to face their parents' disapproval.
Oh, I am willing to agree that the letter sounds contrived. However, that doesn't mean it doesn't or hasn't happened... Sadly, anyone who works in an ER or child services can tell you it most certainly does.