How far away does he need to do surveillance?
This is the question. Because it determines the focal length of the lens. The ultimate low light telephoto is probably the 200 f/2 ($$$), but whether that is the appropriate focal length depends on the subject distance.
Pro-level tripods are big and easy to spot. Leaning the camera on something sturdy, perhaps with a bean bag, may be more practical. Or perhaps a monopod or a very short tripod/stand, like what some sports photographers use for their remote cameras.
The essentials are going to be:
- high res, high ISO camera = Canon 5D3, 1DX, 6D, (or Nikon D4, D800, D600?)
- wide aperture (f/2.8 or faster) prime or zoom with image stabilization, probably a discrete non-white lens (Nikon, Sigma) or white lens with an appropriate dark cover.
- practice focusing; practice steadying the camera.
Depending on the distance, a camera with quiet shutter mode may be of use (5D3, 6D). Also, it will be important to learn how to turn off / tape over anything that lights up on the camera.