But he doesn't own the business. He is a low level employee. He is being paid to do a task.
I think you said the exact right thing here... "He is being paid to do a task"
Is the task being done? If so, he has fulfilled his obligation.
Whether this is ethical or not depends on a lot of things, such as:
-Has the employee been asked to report how much time he is spending on said task? If so, did he lie about the time it takes him?
-Does the employer have policies that restrict usage on the computers, which the employee is now violating because of the extra free time?
-Is the employee being paid on an hourly wage, or a per job / salary or outside contractor even?
What will make it ethical is the details and whether he is being paid an hourly wage for his work with expectations to keep busy by requesting more work if the load is insufficient, or if he is being paid purely on performance, or for completing said work.
If this is a freelance guy, or a guy who is an outside contractor, he has every right to minimize his time spent doing the job provided it is done correctly. Even if he is hourly or salaried company employee, if there are provisions in place that say downtime is acceptable to be doing other things than work related, I think this guy is not being unethical.
I managed a 24/7 technical helpdesk staff in a production environment, and we 100% allowed employees to do college homework, watch videos or pass the time however they wished as long as the phone was answered and the system was inspected at the right intervals and issues were resolved properly. We had a 24/7 requirement on the project, but for our night staff, sometimes they literally was 0 calls and simply monitoring a dashboard for any red values to show up which might not happen the whole night. So yes we allowed the employees leeway on what they could do. If one of my employee were to write certain scripts that helped maintain uptime / recover automatically or otherwise improve the overall performance of the department, (something I frequently did myself also) I would have been quite happy about it.
So yeah, devil is in the details, guy might be acting 100% ethical, or he might be doing something wrong, depending on the expectations of his contract and other things I mentioned above.