Sick cat, need suggestions

A good friend’s cat suddenly began acting lethargic, grumpy and was noticibly limping Saturday night. By Sunday he was out of it to the extent that they took him to a local (very expensive) emergency vet. He was running a temperature of 106 and out of sorts, but they have not yet been able to determine any cause.
He is still at the vet, still running a temp of 105-106, not eating or drinking. They have ruled out Feline Leuk and FIV, and are thinking it might be viral, but so far there has been no improvement. The foot he was limping on seems to no longer be bothering him and there is no sign of injury or swelling. They are giving him fluids and watching and waiting, but are VERY concerned. The vet wants to wait another day or two to see if it is viral and runs it course, then do x-rays, ultrasound of belly and heart and more blood work if necessary… but all without any idea of what they might be looking for.

Does anyone in Doper land have any experience with anything similar that might shed some light on what’s going on, or at least what to explore?

I feel so awful for this couple. They’ve lost two other cats recently, including an 18-year old that she had hand raised.

Thanks in advance for any ideas.

-Jean