Hi
Definitely does sound like a PC issue, i had a squiz through the NAV hotfixes and don't see anything that looks like a fix,
Perhaps try the following,
Have her log onto NAV from a different PC but with her user / pass and test from there, if you have no issues that should eliminate NAV as the problem, then i would suggest that the IT guys try uninstalling and re-installing Excel on the PC
Hope this helps
Neville