It is hard to guess what happened to it without knowing more about how the problem began (was the earliest sympton wilting? any signs of fungal or bacterial spotting before the leaves turned crispy brown?) and advanced (slowly? quickly? From bottom to top? Limb by limb? all at once?). Tomatillos are susceptible to the same diseases as tomatoes, potatoes, peppers and eggplants so there's lots of possibilities.

If the fruit on the healthy plant was pollinated, it should be fine.

Tomatillo pollination is a strange thing. I know people who've only planted one tomatillo plant and got tons of fruit. We've always assumed that this happens because someone else nearby has a tomatillo plant and the pollinators moved from one yard to another, spreading the pollen, or perhaps there was a native closely-related plant whose pollen was carried by insects.

With ground cherries, there are many different kinds, and there's tons of wild ones here in OK. Ground cherries normally are self-pollinating and don't need another ground cherry nearby.