The short answer is no.
You can not access your computer screen using a newtork dvr.
You could point a camera at his screen, but that is the only way you can use the dvr to see his computer screen, and see is all he could do, no keyboard or mouse.
You shoul be able to setup remote desktop, use the same address you use to access your dvr but use port 3389 (you will also have to port forward this in the router). Doing these steps will sllow for using windows remote desktop.
There are many many remote desktop applications out there, for commerical use the no knowledge setup ones (teamviewer, logmein, etc) require you to pay, there are many free options out there (windows rdc, tightvnc, ultravnc, etc) but requires you to have or hire...