Go to MANAGE EVENT > MAIL > SENT > click on the eye icon of an email sent out, right click on the link and open in new tab (keep in mind this will log you in as the person to whom the email was sent to)
In this way you can check to see if the link works and also what the attendee can see if they click the link,
How to help your client:
Ask to refresh the page
Ask to open in another browser
Ask to open the email from a phone or laptop (another device)
Still not working? Ask the support team to call your attendee