Graff approaches Valentine in chapter 9 because Ender has come to place in his training, emotionally speaking, where he cannot (or refuses to) move forward. After feeling completely ignored by Ender and the IF for the many months Ender has been gone, Valentine is insulted that they come to her as Ender's only hope for success at this point.
When she does agree, it is obviously done reluctantly. Valentine does not trust Graff and feels used. She loves Ender but has given in and accepted that his childhood, brotherhood, and relationship with her are forever altered. Her letter is delivered, and Ender probably understands exactly what Valentine understood: they are both being used. The letter seems to work however, as Ender picks up his training and moves on.
The award at the end of the chapter, therefore, is bittersweet. She receives the "highest military award that can be given to a civilian" (153) but the award cannot be made public due to the secrecy surrounding the attack of the Buggers. This is ironic for two reasons. First, Valentine is not flattered by such an award personally. She is uninterested in furthering the military campaign that has taken her little brother away. Second, because no one else even gets to know about the award, she will never receive any public recognition at all. She already feels guilty about helping the IF cause Ender more suffering. This award confirms her feelings and adds to her guilt.
The award is ironic because Valentine's help was contrived and she was reluctant to give it in the first place. She is afraid to help Graff by writing the letter because she suspects it will bring pain and suffering to her brother, Ender. When she receives the award, her suspicions are confirmed. Her letter has helped Ender pass the tests of the game and to go on with command school.