If they're infected with malware, could be a HOSTS file problem or a DNS-redirection problem. Ask them to "ping mail.google.com" in a CMD window and then look up the IP address at the "ARIN: WHOIS Database Search" site at http://ws.arin.net/whois/ to confirm that this is a Google-owned IP address. If it is NOT, then something in their network stack is feeding them bad info. If it IS a Google IP address, then I would disable McAfee to confirm that the problem is being caused by McAfee.
I would first find out if they're behind a NAT router. (Ask them to post the first two octets of their IP address, if they're 192.168, 10.x, or 172.16. to 172.31., then they're behind a NAT router.) If they are, then they're not exposed to direct attacks from outside their LAN if they disable their firewall, and this next step is pretty safe.
As$*u*ming they're behind a NAT router, I would disable all McAfee services from the service control panel and have them go to GMail or Picasa. If it works, re-enable McAfee services and try again.
If they're not behind a NAT router, I would consider their machine to be too exposed to direct attack to disable security services.