Basically, when uploading files through PHP, the owner of the file is whatever Apache is running as. AFAIK, if for example you run Apache as "dudesaccount" and then upload a file, the file will have the owner "dudesaccount". I don't think there is anyway to change this unfortunately (unless phpsuexec can? dunno, never looked into it...).
However, there are ways around this, but you'll have to change the PHP code (and if you don't know PHP, it will probably be a daunting task, you might want to hire someone).
1) During the upload process, you should be able to CHMOD the file as world-readable and world-writable. Do this by using the "chmod" command in PHP, as your script has permission to access the file, then it can change the permissions for other users accessing the file. Obviously not really suitable in a shared-hosting environment...
2) ...or, during the upload process, after the script has copied the file to where it wants it, you can then open it, then open an FTP connection with the user/pass of the website owner, and then use "ftp_fput" to *upload* the file from your script to the server.
Lot's of messing about I know, but that's all I can think of atm...