I would agree that too much sustain muddies up fingerpicking. You can control it to some extent by damping. I think bottleneck needs some sustain; but now as much as Hawaiian style.
I didn't know the necks were metal on the Johnsons! They are wood on Nationals. If she is not dead set on a metal one, Steve, I would suggest looking at the Regal which is a wooden resonator guitar (similar to a round-necked Dobro.). I played one recently in a shop here and it sounded great with a metal slide on my finger. It sells for about the equivalent of 500 US dollars here, so it is probably cheaper in the US. I am told it is made by the same factory that makes Samick guitars.