Are you sure your friend's shot is just 1 shot?
I doubt so, since I can't even have it in one shot with a 135/2.5 on my 5D
So your friend certainly did not do it that 'easily'
Read up on the Brenizer Technique as suggested.
The Canon crop factor is 1.6x.
Pentax is 1.5x.
So for the same focal length lens, Pentax will actually be nearer for the same field of view (FOV), so less DOF.
But then, your lenses are different focal lengths.
The 85/1.8 on a D1100 say at a working distance of 6m to do that shot you posted will have DOF of 0.17m
The DA70 at f2.4 for the same framing, will be at a working distance of 4.6m and DOF about 0.21m
Close enough in DOF shallowness for a 1 frame photo.