The license.pvk is a private key to a certificate. Like any private certificate key, it cannot be recreated from the public key data.
I suggest they should exhaust every option in finding their .pvk file, including checking any system backups and user profile areas they may have available.
If it truly is gone, I suggest you open a support incident to talk through your options.
It may be possible to reinstall Bigfix and restore your old database with a new key pair, but your existing clients will not communicate with the new deployment until you do a ‘masthead switch’ - replacing the actionsite.afxm on every endpoint. That’s much easier to do while you have the old deployment still up and running, as you can send an action from the old server to switch clients to the new one.