I don't think there's anyway the stock derailleur would work. It has a stated capacity of 31T (what you get now with a 11-42 cassette). While Shimano is known to be conservative, and you could work with a somewhat larger cassette, I don't think you could push it to an 11-51. Maybe that Garbaruk cage delbiker1 linked would work. But aside from the cage, you need pulley wheels, though maybe you could use the wheels of the existing derailleur.
I think the reality is your friend should have gotten the the 2x instead of the 1x setup. While a smaller ring upfront would give him a lower end, even a 40-11 combination isn't a lot of top end as it is. If you reduced that to be the equivalent of changing the cassette from a 42 to a 51, you would need a 33T ring and top end speed would be about 25 MPH at a cadence of 100 RPM. Plenty high on the flat for a mere mortal, but they would spin out on most any fast descent.
Of course, this all depends on how they are using the bike. If they are really doing mostly off-road and more trails than fast gravel or paved roads, then maybe they don't need much top end and a smaller chain ring is the simple solution.