Rory has just wrapped Giles extraordnarily tightly around her finger. And he'd already do anything for Xander. Not to mention the soft spot for Marty (and his pine trees!) Giles would concur that the young 'uns need to cut loose, so if you want someone whom Angel would ever suspect...

He's doing it for Rory, Xander, Marty, et al, of course, not Angel, but he'd still do it. (And then melt quietly out the door. Perhaps to meet Jenny for dinner...)