NCB – I have never heard of someone forgetting to light candles (BT or FFB). That would be like forgetting that Shabbos comes every Friday night,in which case, the person is apt to do a lot worse things than forgetting to light candles.
I suppose people who have Alzheimer’s might forget to light candles, but they usuallly have caretakers to remind them. And if they forgot anyhow, I would guess that the Psak would be that they don’t have to light an extra candle (but their caretaker should ask a sheilah).