Who is this camera friend... he either doesn't know much about the physics of photography, or can't communicate it to you well.
Having shallow focus is the choice of the director in discussion with the DP. It's meant to focus the viewer's attention and support the story by doing so. When doing a moving shallow focus shot, you'll need a second person attached to the camera to perform the follow focus. Focus isn't a sharp image, it's a specific point in space at at specific distance form the capture plane of the camera (the chip/cmos/ccd)... it is the single point at which the light from that point converge on the capture plane. Everything in front or behind it are out of focus... period.
When you turn the focus ring on the lens to infinity, it is the farthest out you can focus away from the capture plane. there are chunks of space before and after the singular point that are in "acceptable focus"
3 things make up the Depth of Field (DoF) of a shot:
- The f-stop or aperture (ratio of the the size of the hole light is coming through;iris to the length of the lens)
- The focal length (the mm rating of the lens -- like 18mm-55mm on a short zoom lens)
- The size of the capture plane (full frame DSLR = 35mm, my canon XL1s ~ 8mm)
if you make the aperture smaller, you need to increase the amount of light coming in... however, you lengthen the "acceptable focus" range... also known as a deeper DoF. It is easier to make your focus adjustments if you have more fudge room than if you have less, the drawback is that there is more stuff in focus... and if you need the background to be out of focus for a shot, the movement of the camera may have to change, or the production will need a 1st AC (follow focus operator) to do nothing but monitor and adjust the distance from the camera to the subject. IMHO, this is the hardest technical skill in the camera department, there's no close enough, it's right or out of focus!
The focal length magnifies the background and compresses space as it gets longer. An 18mm lens (or zoom set at 18mm) will seem to have a much deeper DoF than an 85mm lens. I say seem, because it's really doing this:
A) Camera ---------- 8' ---------- Subject ---------- 8' ---------- Background
then magnifying it (not actually moving the stuff, just zooming in) to appear closer to the camera:
B) Camera ----- 4' ----- Subject ----- 4' ----- Background
The distances now look like half each, but if you look at it thusly:
A) CS = 8'; CB = 16'
B) CS = 4'; CB = 8'
you'll see that the subject has moved 4' closer to the camera whereas the background has moved 8' closer. So the slightly out of focus background becomes larger and you can see it as more out of focus than it seemed before (google: Circles of Confusion).
the bottom line here is that changing the lens doesn't undo the physics of getting the shot you want... if you want a shallower DoF for a shot, he'll need to have a 1st AC to pull focus, or work really hard to keep the distance from the camera to the subject constant to keep them in focus so he doesn't have to pull focus on the shot... or you'll have to use a deeper DoF.