It has nothing to do with windows.
I dont know the solution but you can try Deleting your GTA San Andreas User Files folder, not sure it will work; i had a similar problem and deleting the folder worked for me.
EDIT: Try using other the CTRL button
Actually it could have to do with Windows. If what is happening to him is what I think it is, it may be an odd bug in Windows' permissions system that is resulting in his computer not allowing GTA:SA access to the computer's clipboard while running SA:MP.
Deleting the settings wouldn't fix this issue he is having, as the SA:MP chatbox is a memory hook, and is not loaded as a part of GTA:SA's settings itself. Deleting the folder is also a bad idea, because you are deleting everything in it. Your method likely worked for you because SA:MP may store a file for its own settings inside the folder. In that case, the SA:MP settings file would be the only file you would want to try to delete, rather than deleting everything for your game's settings entirely.