And what help will you get from knowing the miner is indeed a genuine S9?
If the seller is refusing to allow your friend to test is like buying a car without the owner letting you open the hood and see if there is a still a motor inside.
If I buy a used thing and the guy selling it refuses to power it up it's good bye and bye bye in seconds.
Probably you're still thinking about the deal because he offered you a very good price so you want to take the risk.
Sorry but this is another red flag!
Stay away and search for another seller!