[He didn't know. It never occurred to him that her friend is still alive. He was given the impression that death is the reasoning behind her wanting to forget. But it isn't. And to him, it's a blessing more than anything else. At least in her case, Katniss has a chance to return to him, if she is willing to wait.]

Then you have something to look forward to. You may not know when, but you will see him again, whether in your world or in Luceti.

[If anything, knowing that should bring her hope and some peace of mind. And if it doesn't, then Oak isn't sure if he could help her at all. But he's not going to throw in the towel just yet. It all depends on what she says or do next.]