In Goblet of fire, chapter 23 (Yule Ball) we hear Hagrid talking about his family for the first time.
“It was my mother,” said Hagrid quietly. “She was one o’ the las’ ones in Britain. ’Course, I can’ remember her too well . . . she left, see. When I was abou’ three. She wasn’ really the maternal sort. Well . . . it’s not in their natures, is it? Dunno what happened to her . . . might be dead fer all I know. . . .”
And then we get some more details from Rita Skeeter in the next chapter (Rita Skeeter's Scoop)
His mother, we can exclusively reveal, is none other than the giantess Fridwulfa, whose whereabouts are currently unknown. . . . While many of the giants who served He-Who-Must-Not-Be-Named were killed by Aurors working against the Dark Side, Fridwulfa was not among them. It is possible she escaped to one of the giant communities still existing in foreign mountain ranges.
So we know that neither Hagrid nor Harry, Ron or Hermione know the fate of Hagrid's Mom. And then in Order of Phoenix, chapter 20 (Hagrid's Tale) Hermione says,
“Hagrid?” said Hermione quietly after a while. “Mmm?” “Did you . . . was there any sign of . . . did you hear anything about your . . . your . . . mother while you were there?”
Naturally after hearing where he had gone she was curious to know if he managed to meet his mother. And then immediately after that she says:
Hagrid’s unobscured eye rested upon her, and Hermione looked rather scared. “I’m sorry . . . I . . . forget it —” “Dead,” Hagrid grunted. “Died years ago. They told me.” “Oh . . . I’m . . . I’m really sorry,” said Hermione in a very small voice.
She says "I forgot". But when did she learn about the fate of Hagrid's mother when till the fourth book they were not sure if she is dead or alive?