The Lyme's disease screening...in fact all the tick-borne disease (TBD) screenings...are inaccurate--lots of false negatives. Plus there are newly emergent TBDs that they don't have tests for. It might be worth a course of doxycycline to see if there is any improvement. If there's no improvement after a couple of weeks, she can always stop the doxy. If she tries it, she should add a probiotic to Conner's food. The doxy can be really hard on the intestinal fauna.
It's not unusual for the symptoms to appear suddenly and cascade as happened in Conner. Symptoms vary, but the three he showed (lethargy, no appetite and lameness) are classic.
I sure hope she can find an answer soon! Poor boy!!
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."