I have a php script that will pull in information from a csv file (save the excel as a csv format) and populate a mysql db with it. I imagine one could be able to find one that would pull from the database and write a csv formatted file that excel could then open as a normal spreadsheet.
I will post a link the importer script below:
http://codewalkers.com/seecode/175.html
Some export examples/info I found:
http://www.modwest.com/help/kb.phtml?qid=135&cat=6
This one below is not oss or free, but looks pretty powerful:
http://www.comdevweb.com/csvdatatool.php?code=CSVIM31
http://www.zend.com/codex.php?id=1036&single=1
http://www.webmasterworld.com/forum88/5504.htm
You could pull the csv into the mysql db, then allow the clients to use it in a form that pulls/adds/modifies/deletes the data that is necessary, then write it to the database, then create the csv file by pulling it from mysql and email it using the mail function. Each row in the csv file would become a mysql row, so it would use the same basic loops that you use for mysql I would think (as far as stepping through each field to read/write the csv file).
Of course, this means that the user that php/apache runs as would have writable access to a directory to create the csv file - which may open a security hole if done improperly.