You want your friend to lie on official forms and get involved in your tax evasion? On top of the person you bought it from failing to register it in the state within 30 days as required?
Well, advice #1 is don't buy a foreign registered vehicle from a private seller within the state, and certainly not one that has been here for so long unless you want to risk additional fines or confiscation. They can and do look for evidence showing when you brought the car in, most people will show a corresponding ferry ticket.
Advice #2 is don't try involve your friends in tax evasion. Having someone else front the documentation is illegal.
Has anyone actually tried this with a UK to NI import post Brexit? I am looking at a 2020 car that was imported to NI in 2021 and has been in private NI ownership since..