In the old days
I too have no idea what the “sweat test” is, but in the old days, one way to help diagnose MS was to put the patient in a hot bath. While not definitive, if s/he became significantly weaker quickly, it was taken as an indication of MS. The reason is that nerves conduct more slowly when warm. With the damage to the myelin already slowing conduction, the change caused by heat would slow conduction to the point of weakness. Maybe the doctor is thinking along those lines.
Diagnosing GBS is not as easy as it might seem. In another thread, there is a link to a presentation on GBS given to the Transverse Myelitis Association. The clinical presentation in both diseases is very nearly identical, to the point where most TM patients are initially diagnosed with GBS.
By the way, a single test (high protein level in CSF) is rarely sufficient to prove a neurological disease. Other tests (other than the sweat test, I think) should be done, and done soon.
Godspeed with this. I hope you friend has gotten treatment by now.