Earlier quoted context omitted.
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. Bouloche has been corrected.
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)
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.