"She is writing to our son."
Translation:Elle correspond avec notre fils.
Actually, if we take only the meaning, it's fine, because the verb "correspondre" means "to have a correspondence with someone".
But I have to agree that I would also translate this with "Elle écrit à notre fils", since there is the verb "to correspond" for "correspondre", and in this specific case, word for word translation would not hurt at all.
That's my opinion anyway.