locked theme editor

hi – i am a beginner wordpress user. a friend of mine installed wordpress on his hosting site to which i have access but am not super-familiar with. when i launch wordpress on an internet browser and bring up the theme editor, i can’t edit any of the themes and there is a message below the box where the code is that says: “If this file were writable you could edit it.” my friend who uploaded the themes claims that he did it all with an FTP client and had no problem / did not know the site was locked.

I would love to know how to unlock this. do i do this in wordpress or at the hosting site? any hints on where to look in a hosting site?