The next sentence tells us what Harry thinks about Bathilda, and then it shows us what makes him think this way.
She seemed to have forgotten that she could do magic too, for she lit the candles clumsily by hand, her trailing lace cuff in constant danger of catching fire.
Again what Harry notices is decay: Bathilda has forgotten her powers, she has become physically clumsy, her old clothes have lost their shape, and she could catch fire and die but she hardly seems to care.
There must have been something wrong with the electricity, because the lights flickered momentarily overhead, the bright yellow filaments fluttering and dying like burning moths.
She seemed to be under the impression that the crew were asleep, for she continued to call loudly for their help, her desperate voice echoing off the empty decks.
He appeared to be disoriented, looking around in a daze for his friends, who had long since disappeared into the concrete tunnel at the end of the platform.