A vehicle deemed a total loss ("totaled") by an insurance company results in the issuance of a salvage title for the vehicle. It becomes a "salvage vehicle" and CANNOT be driven on the highways or have a valid license plate. It can be sold ("AS IS") to an automobile rebuilder to be used either for parts or to be restored ("rebuilt"). If restored, it must be inspected and ultimately a "rebuilt title" will be issued for the vehicle. A rebuilt vehicle with a rebuilt title CAN be driven on the highways.
Your friend is not obligated to tell the adjuster anything. Your intentions to buy the salvaged vehicle should have no affect on your friend's claim with his insurance company. These events are not related to each other. However, it may be to your benefit to contact the claims adjuster to tell him of your intentions to buy the vehicle. It is possible that you can negotiate something with the insurance company. Your friend is going to be entitled to the fair market value of his vehicle and it will be the responsibility of his insurance company to 'dispose' of the vehicle.
You can get a car driver insurance new quote here to see how much a policy will cost.