...Alright. Let me just get the explanation out of the way first.
John's eye has allowed him to do a few things that are relevant here. The first thing is, he can find a person's (or being's) death and attack them with it directly. The second is that he can find a potentially infinite number of things at once (evidenced by the time he found "all of the things that could go wrong with a motorcycle"), and the third is that he can use his gift to see across all of space-time and every timeline contained within.
The other thing is that John's eye explicitly shows him the nature of everything he sees. When looking at someone or something with his third eye, he's seeing the reality of that person or thing:
I concentrated, raising my gift again, using it to study the reality of the thing before me, opening up my inner eye, my private eye, to See the world as it really is. And then I almost dropped the shard as I realised what it was I was holding.
"What?" Bettie said excitedly. "What did you See?"
"Temporal energies," I said. "This mirror shard is soaked in Time, in Time magic. I can actually see inverted tachyons, shooting up and down the broken edges."
Bettie gave me a hard look, only slightly spoiled by her pouting mouth. "Yes, very nice, darling, very dramatic. But what does that mean?"
"It means, I know what mirror this came from," I said, pulling a handkerchief from my pocket and carefully wrapping up the vicious-edged shard before tucking it very carefully into my coat pocket. -The Bride Wore Black Leather
With the above in mind, I think it's possible that John will open his eye, see Nerif's nature as a being that exists in every timeline, find all of his alternate selves, and then find all of their deaths. John would be left weakened from the effort, but he's lived through using his gift so widely before (see the aforementioned scene where he found "all of the things that could go wrong with a motorcycle" and enforced those possibilities on a bunch of bikers), and he's also shown to be capable of moving and fighting after doing something like that.
