As has been said, there are two answers to this.
1. Get your customers to install a virtual machine to run windows to get IE to work, or use a product based on WINE to run IE
2. Get your site to work with modern web standards, as that is how things are going with more macs, iPads and android devices accessing the web.
Not being mean, but its a big ask asking customers to jump through hoops installing windows compatibility when they just want to access your services. I know I would not want to spend an hour or so installing a work around when I could spend less than an hour looking for an alternative service.
Best the company does the work for the customer, rather than expecting the customer to tailor their own setup to meet your company's requirements