I don't think we should get too distracted with sand/dirt down the intake. Yes, not good and shame on Honda but it's not like he ran the Baja with no air cleaner. Only a compression check will tell us if debris killed that cylinder.
We know - 1. The fuel is good enough that it will run on one cylinder. 2. One cylinder definitely isn't firing, he can hold his hand on that exhaust when running. 3. Something is wrong with that injector or its wiring. I asked earlier if Honda fixed the breaking injector wiring at the plug end or if it's still happening to some recent models, 2021 in this case.
Are the two injectors identical, can he switch them to see if the problem follows the injector? The wire cannot be switched with it because of the 270* firing order, so the injector wiring for cylinder 1 has to stay on cylinder 1 and 2 stays with 2.