XAMPP is nothing more then a combo-package for Linux or Windows containing several modules, being the Apache webserver with PHP and Perl included, and MySQL server for a back-end database. What you do with that, is completely up to you..... but if you have that as a base as your server, it is likely that what is running now, is a completely customized piece of web-based software..... Am I to understand you are wanting to replace whatever is running on your server with XAMPP now?
Sounds more like he wants an entire invoicing system / software package.