This happened once when someone could access the portal in the United States but couldn't access the portal in Pakistan. From our end, Salesforce does not restrict access based on country location. Therefore, it should work. The best we can do is to reset the password and see if that makes a difference. This may have something to do with their network.
If the person continues to have issues and is working with other researchers who are in different locations, recommend that one of their colleagues complete the application on their behalf.