You're being harsh for pouls/poux being singular/plural, provided it was the first time the author had encountered those words. That kind of knowledge comes with experience with the words, which the author didn't have.
I still think it is weird to be that bad after 15 years in the country.
Counter point — I think it was French ambassador or someone from diplomatic corpus who impressed me in their interview on the radio. I think they have been in the country for half a year only and spoke amazingly well. And I mean it. It was not amazing in the way bear dancing is amazing (e.g. it dance terribly, but the fact itself is amazing). She spoke genuinely very well. And Lithuanian is a very very difficult language.
I don't know what foreign learners of french learn, but native speakers very early learn the complete list of words ending with ou whose plural is with an x instead of an s, which poux is one of (there are only 7 of them)
Keep in mind that her encounter with "pouls" was spoken, not written. The "poux" warning probably came with a written note.
Regardless, @bambax said she should have known that, in a spoken context [lə pu] is the pulse and [lɛ pu] is lice. I argue that it isn't surpising that the author didn't have that knowledge. Lice exceedingly rare in adult life (if common for school kids). Pulse is specialized vocabulary.
Bouloche has been corrected.