Hmm. Maybe whatever he did can be fixed. What exactly did he do? If you can't answer that, then list what channels show up on the Wii menu, and if Homebrew Channel is installed. (it should be if he did a softmod, aka Software Modification) If it is, open it and list the apps in it. You can tell a lot about what was done by the apps in HBC, unless he took the SD Card, which he probably didn't.
The step after that would be to plug the SD card from the Wii into the computer and put
Syscheck on it, and run it through HBC to look at the Wii's internals and what was modified.
Only a small variety of things can effect disc loading. What seems likely to me is that one of the IOS that some of your games run on were modified or uninstalled. Mario Kart Wii runs on IOS36. He might have installed a broken IOS36 over it. Should be an easy fix with a wad manager and a wad of IOS36.