I hope you've found a solution. But here is my experience :
We face the same problem when employees have network restrictions . In this case, the problem is that some employees are not allowed to watch streaming videos.
Similar to what Claire mentioned this could be related to the network connection. For Hosted and On-prem, the allowed ports must be 80, and 443. You must also either whitelist by domain or whitelist by IP address.
Or if the user was using LTE instead of wifi, you might have them disconnect from the network wifi and reconnect to it which should ensure its connection.
If the issue is specific to that specific user you might want to review their user permission groups to see if there are any differences with other users.
If you have confirmed the network connection and the customers permission group. I would recommend creating a case in your private support group. This will allow support to further review your site and your settings.
Hope this helps.