This usage is probably in violation of the Master Service Agreement. I understand about saving costs, but if your usage is in violation, and you're found out, they can retroactively charge you for estimated license cost.
The good news is that, in many cases, you can still cut costs while not using full licenses. You can check out this help topic and related links to see if you can find a more cost efficient license. For example, maybe they only need Community access, or maybe a Portal User license.
Also, speaking directly about this setup, using a single user record creates all sorts of issues, like trying to run reports, figure out who did a certain action, etc. Salesforce really does work best when you have a 1:1 ratio of users to licenses.
Consider having a conversation with your AE regarding this matter.