He can check if there are any fonts inside of \Grand Theft Auto IV\IVMP\CEGUI\fonts
There should be 5 of them
If that is okay, then just do what Osamad said.. when entering IV:MP, go to Settings>Chat
Set default font as "tahoma-bold" or any font from the before mentioned location with fonts
Be sure that the values underneath are all 0, font size.. at least 10