BTW

These laws have been used to sieze the assets of various people under the Comprehensive Forfeiture Act of 1984 , under this act it makes the responsibility of owners quite clear. The "owner" must be pro-active in ensuring that thier property is not being used for illegal purposes and must take any steps to prevent it.
A yacht owner had his vessel siezed under these laws after his boat was used for a party and a remnant of a joint was found . it was decided that since he "allowed" the vessel to be used for illegal purposes he must face the consequences. The courts decided that since the owner had not carried out sufficient checks it was his inaction that "allowed" his vessel to be used for the illegal activity of toking.

You would be placing your father in a very similar position

Ignorance of the law is not accepted by courts.