Adding a developer for editing the client application with a single user license

Adding a developer for editing the client application with a single user license

Hi,
I want to know that I as a developer I developed one application and handed over to the customer who is using the application on a single user license.
Now after6 months customer came back to me and needs some changes in the application.
Can a customer add me as a developer so I can edit his application even though he has only single user license?
If not, then what is the way to do it?
Developer mode is not suitable to make changes and push the changes as developer mode doesn't support all the features which are available in normal license login.
So I need suggestion or understanding how to fix / enhance existing live application of client when he has only single user login. I cannot use his login because the application is being used whole day, it is live.
Thank you