I don't know  if your employee is still not able to connect or not but here's my experience.

I already have an Intuit/QB account so I didn't have to create one. I tried to use the activation link on my PC in a Firefox browser. I got the, "Sorry, your employer no longer has a QB Workforce account..." I tried the link on my phone, which also uses Firefox as my default browser, and it worked with my existing account.

If they can't do it on one device, have them try another.