Have that user try logging in with a private browser/incognito. Typically a 500 error is on the server side and not the client side (user's computer).

You said you called the website host, is this a third party system that many people use like https://www.adp.com/what-we-offer/payroll.aspx or https://www.paychex.com/?  Or is this something that you host on your own and have control of the server?

If they are still getting the error, you might try and hit f12 to open up the browser console. Look at the console tab for any client side errors that could be preventing something. Look on the network tab then , click on some files to see what is going on in the background. That could help them diagnose the error but generally a 500 error means you are not getting any details.

1) As @Scott mentioned, 500 Internal Errors only occur on the server side.

To fix this, you'll refer to your server logs to find + fix the problem.

2) You mentioned, "We called the website host and they don't have anyone else complaining of this issue."

Correct.

This occurs in only 1x code base.

Said differently, your Webhost will only receive support issues from people using your code + Webserver config.

So... only people using your code can possibly see this problem.

500 mostly involve a reverse-proxy (that is by definition the host side) not able to reach the underlying hosts/services in a timely fashion.
It might also happen in web application chains because then the requests are mostly proxied to an execution backend (php, etc.  through fcgi))
500 error's actually tell the user there is no backend to service the request.

The user tried again a few days later and it worked.  I have no idea how it resolved.  The hosting site is a payroll company who hosts their own server.  

Strange that she was able to finish her work that day on another computer in her office.

Hard to tell even with access to the logs if it happens while looking into the system. Without logging available and being at a distance there also near 0 chance a cause can be determined.

Error 500 ALWAYS that a Proxy/Loadbalancer (or webserver proxying a request)  cannot access some backend system.