convert Sit file to a Zip file

You can't convert one compression scheme to another. You need to decompress the Stuffit archive and then zip the files that come out of it.

You can use the free Stuffit Expander to open the .sit file. Once the files are on the desktop, compress them with the zip function built into OS X.

Can you change the backup's choice of file to something else?

He could download a commercial application for Windows to unpack it, via this link.
http://www.fileinfo.com/extension/sit