I believe that the Defender Offline tool is a bit more than just MRT.exe. Defender Offline runs before Windows even starts up. That lets them go after rootkits and trojans that could be hidden from MRT.exe.
My acquaintance has Automatic Updates turned on. The problem is that he installs stuff without understanding what it is. Even a good AV can't completely protect against that 100%. Whatever has his machine has blocked all access to AV sites and Microsoft updates. I'm hoping this tool will set things right. Otherwise I'm going to have to nuke the thing from orbit and start over. ;)