「雪子はご飯を食べている間テレビを見ていた。」 This sentence makes perfect sense. Yukiko WAS watchING TV, past progressive.
However, 「友達が今日来ると言うことをすっかり忘れていた。」 is translated in ADoBJG as "I completely forgot the fact that my friend is coming today."
"Forgot" is of course is past tense, not past progressive. It doesn't even make sense to say "I was completely forgetting that my friend said he was coming today." Except maybe in the larger context of "When I promised my mom that I would help paint the living room, I was completely forgetting the fact that my friend said he was coming over today." That's the only scenario I can imagine that working in, so what's going on here?
To illustrate and belabor the point, if you were to say 「彼は死んでいました。」 there is a huge difference between "He is dead." and "He is dying." Any help would be greatly appreciated.