Just to clarify, do you mean that you and your friend are sharing a hosting account? If you were sharing a domain, the domain would be the same (not two different domains)? If you are just sharing a hosting account, you will want to make sure you set up your domain as an Addon domain so that it has separate DNS and a separate document root (folder).
Unless you are using a WordPress network (formerly Multisite), you should have two separate dashboards (one for her site and one for yours). Simply make sure you log into your dashboard (should be FaustEffects.com/wp-admin) to make your theme change.
If by chance you are using a network, by default, theme changes apply to all sites in the network. However, there is at least one plugin that allows you to have separate themes:
Hope this helps!