Besides telling them to visit clueville, I wouldn't be very into the job unless the money was just that good. If the webserver is on their LAN/dmz, you might look into converting the data to MySQL and setting them up with ODBC access to it. So then they can dork away in Access. Basically get to the bottom of why they feel they need the _data_ stored in access. Sure, if there are other apps that you don't want to mess with [that use this DB], then it might be less trouble to just leave it in access and go ODBC.
Check out PEAR::DB and ADODB.
http://php.weblogs.com/ADODB
http://pear.php.net
Both should provide generic SQL functions, so you shouldn't have to change much to get it working with access, then possibly move to MySQL in the future. Just stay away from db-specific things like MySQL's LIMIT. Use, for example in ADODB, the $db->SelectLimit($query, $n) function to convert your SQL into proper LIMIT/OFFSET syntax for that specific engine.
-r