He needs to reboot his computer. That should fix it.

It is quicker just to logoff and logon.

I have the same problem  here.
I've found this. Maybe this is worth giving a try:
http://nickporter.spaces.live.com/Blog/cns!1p9xP44krkAbpdib2DZRYu-Q!151.entry

Make sure that this person has your portals, or whoevers, in their
trusted sites and also that the have "automatic logon with username and
password" selected under IE if that is their browser. You can get to
this via - tools > internet options > security > custom level > scroll
to bottom and select what I mentioned. You probably want to do this
only on "trusted sites" but you can also do it on the "internet" option
as well.