Photo editing doesn't need much at all, but video editing can be demanding, depending on how sophisticated the editing he's doing is.
I'd say definitely Skylake, i5 or i7, with Win10, 16GB of memory. Video editing tends to use a lot of hard drive space, so a 120GB or 250GB Samsung 850 EVO SSD and a 2TB or 3TB hard drive would be a good place to start. Figure a regular mid-tower so there's room to add more if desired. The monitor has to support the highest resolution of video he wants to work with, so if he wants to edit videos at 3840 x 2160 (4K) then the monitor has to support that. More than 60Hz shouldn't be an issue for what he's doing. If the most he needs is 1440p, then I really like the Asus MX27AQ 27". For editing he'll want a nice mouse. I like the Logitech G502 but he may have a favorite of his own.
Depending on your budget, this can be fine tuned more.