I agree with Arthur, in the last week I have noticed that Snap/OvrC is struggling to load pages in Firefox. Have them try a different browser first, if it doesn't work call Tech Support for them. They are paying a Connect fee for a reason.

Have them try from the email link in a different browser, or incognito/private mode and see if that helps.

Otherwise, remove the customer from Manage Access list and send another OvrC Connect user invite again.

Just some ideas if you haven't gotten it to work yet.