Isn't Helium one of the easiest elements to purify? Just cool it below 14 Kelvin, which will make everything else freeze out. Collect the remaining liquid, which should be pure Helium.
My point is that there's "maximally efficient / profitable" versus "can be made available as an emergency alternative".
Cooling to 14 K isn't the cheapest option, but it has very low complexity. You can "simply" pressurise the source gas, cool it to room temperature through an ordinary heat exchanger, then allow it to expand. The only issue is that if you do this naively, the expansion nozzle will get clogged with ice.
Obviously, this wastes a lot of Helium, but we have lots of it. If what's needed is high purity Helium, then throwing away even 90% to get 10% that's 6N pure should be no problem for an industrial nation.
However, any air (or gas) liquefaction / separation plant that is already making purified industrial gases from air or other sources could be adapted in a matter of weeks or at most a couple of months.