A Question For Premium Theme Authors

Ok, so here is the theoretical scenario. Theoretical because I don’t release or develop themes or plugins.

I’m a person who has created a plugin which modifies an integral part of a WordPress theme. End users end up contacting me to perform custom work with regards to implementing my plugin or their customizations to my plugin into their theme. However, in order to edit their theme, they must send it to me. This is where I had a few questions.

For you premium theme authors out their who don’t license your themes under the GPL, is it within your license for your customers to send me their theme for me to work on? If you’re not under the GPL, wouldn’t this violate their theme license because they have redistributed their theme to me in order to work on it? Am I required to immediately delete the theme once I have shipped them the finished product? Unless I am redistributing the themes my customers have provided to me to edit, how would you ever know your customer sent me their purchased theme?