To answer your questions easily, both are no. However, you do have some options on the user who forgot to borrow. You could change the application over from Network to Stand-Alone and run it as a 30-day trial. This would work only once, but is useful in emergency situations. The best practice would be to allow the user to connect to the NLM over a VPN connection, assuming you provide this service.
As for your second question, it is simply no. The user must be connected to return the license. There are ways to do an administrative forced return of a borrowed license in cases where one gets stuck. However, these options are not supported by Autodesk.
There's nothing illegal about it, it is just simply converting your existing installation from a networked install to a stand-alone trial. You would not be able to activate the stand-alone trial because the serial number is a network seat. This isn't approved by Autodesk however, but you're not going to be audited for it.
Stand-alone seats will run for 30-days without being activated. After that, they can no longer be run as a trial and need to be activated. In this case, you can convert the network seat over to stand-alone and run it as 30-days. When the user returns, simply revert back to network seat. Once this has been done once, it will never work again as stand-alone on that machine. So, I consider it an emergency fix.
To do this, you need to modify the registry of that user's machine. Open regedit and go to the following key:
HKEY_LOCAL_MACHINE\SOFTWARE\Autodesk\AutoCAD\R17.x\ACAD-xxxx:409\AdLM
(Note that the x's are dependent on what flavor and version of AutoCAD you have installed.)
Locate the REG_DWORD for Type and change it from hexidecimal 19(25) to 2a(42).
The product will run as Stand-Alone now as a full trial for 30 days. When the user returns, simply change back from 2a to 19. Done.
Now invest in some VPN connectivity for offsite users so they can access the WAN and your NLM!
Good Luck!