Do you have or can you request a copy of the firewall logs that indicates where your husband has allegedly surfed on their network?
There are several possibilities, but take with a grain of salt until you (or an attorney) can gather more evidence:
1) Your hubby's logon password was something stupidly easy to guess and/or he put on a sticky note with his password around his desk. Someone gets password, logs into his PC or user account, surfs pr0nz, and profits. IT departments' logs may go solely by user name basis, rather than computer.
2) Same as #1, but add that someone could have logged into his PC remotely. Windows Remote Desktop locks the host computer while the guest accesses it, and the host's monitor can be in a screen saver or off.
3) Malware can certainly cause deh pronz to appear in firewall logs.