TweakUI should do it.
There's a 3rd party equivalent.Unless he is on Vista, in which case there isn't a TweakUI for Vista.
There's a reg key you can change to stop it, quick google:
System Key: [HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\
Explorer]
Value Name: NoLowDiskSpaceChecks
Data Type: REG_DWORD (DWORD Value)
Value Data: (0 = default, 1 = disable messages)