"She WAS strong and healthy when she was 20." << simple statement about a moment in the past. "She HAD BEEN strong and healthy before she was paralyzed in a car accident." << Two statements about two different moments in the past. "She HAD BEEN strong and healthy." << A statement about the earlier of two moments in the past. The later moment is implied or already known. "She had been strong and healthy." << is equivalent to "She had been strong and healthy [but she wasn't any more] [because of an already known past event or an implied past event]. The example "She was a woman of 50 who had been obviously pretty once." is equivalent to "She had been pretty when she was young but wasn't pretty at 50."

You use the past perfect because the story is already in past tense, and she was pretty at an earlier time, before the moment that the speaker is describing. Use past tense to describe an event that took place in the past, and use past perfect to describe something that happened even earlier. "My father visitED me yesterday. He HAD tried to visit the day before, but I HAD been out of town then." (And, as Steev says, the final word should be "once," meaning "at one time [in the past].")

I guess should read, "who had been obviously pretty once." The meaning is that at one time she was pretty and this is obvious, (presumably by looking at her.)