Is the user blocked from getting to google servers somehow? Maybe a proxy setting or dns/host file settings possibly? Since it's happening with multiple browsers it seems likely to be something system or network related rather than browser setting related.
Specifically, the user has to be able to access this:
as well as:
(actually the path goes into several folders on there, but the path changes frequently).
Is it only a problem for the one site or all sites using reCAPTCHA? Try this site:
The captcha is at the bottom - don't register, but just see if the reCAPTCHA shows up correctly.
Maybe a screenshot of what's happening would help.