I don't know if you've seen the end of the season yet, but
Donna ends up becoming the "DoctorDonna", part human and part Time Lord, and saves the multiverse from
Davros' plan to destroy it with the "reality bomb", but then to save
her from insanity she has to be turned back into who she was before,
and not only that, has to have all her memories of traveling with the
Doctor be erased.
This is a pretty memorable fate for a companion, so I think it's fair to assume that the Doctor told River about this at some point, and keep in mind that the River in "Silence in the Library" and "Forest of the Dead" was at the end of her time with the Doctor (as explained in 'Forest of the Dead'), so anything he had ever told her, that version of River would already know.
This was just one of several bits of foreshadowing in that season that something big was going to happen to Donna, for example at the end of "The Planet of the Ood", Ood Sigma said:
And know this, Doctor Donna. You will never be forgotten. Our children will sing of the Doctor Donna, and our children's children, and the wind and the ice and the snow will carry your names forever.
And in "The Stolen Earth", an albino-looking alien in The Shadow Proclamation said:
ALBINO: You are something new.
DONNA: Not me. I'm just a temp. Shorthand, filing, hundred words per
minute. Fat lot of good that is now. I'm no use to anyone.
ALBINO: I'm so sorry for your loss.
DONNA: Yeah. My whole planet's gone.
ALBINO: I mean the loss that is yet to come. God save you.