[ She doesn't need to explain what a Giles is. It's all too easy to interpret from her tone alone and he knows exactly what it's like to lose someone like that. Gideon isn't dead but he might as well be after that disappearing act and as if that wasn't enough, what she says next is practically an echo of why his own mentor left.
At a loss for words, Reid hesitates for a moment. They're not really that close and maybe he's overstepping his boundaries here, but that's a risk he's willing to take and so he steps in close to pull her into a slightly awkward hug. ]