That seems to be a glaring error in the 2019 Instructions.

I think it should say that the spouse does NOT elect to be treated as a Resident Alien.

Here is the 2018 Instructions that show it correctly.

https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf#page=16

So yes, the client can claim Head of Household if the spouse is a Nonresident Alien (that does NOT elect to be treated as a Resident) and meets the other criteria for Head of Household.