When Kosh was poisoned, Dr Benjamin Kyle saved him. How could a human doctor be able to save a Vorlon when the Vorlon biology is so different from humans'? In fact, the Vorlon had no physical form and seemed to exist in some kind of floating gaseous form like a ghost. How can that be even treatable in the first place by a human doctor?
Not easily, but clearly it was done.
Both doctors on the show are specialists in xenobiology, which means they have quite a bit of experience in figuring out how other species work. Admittedly, Dr. Kyle was working from scratch, given the lack of info on the Vorlons, but thats likely to be a situation he's handled before.
The majority of Dr. Kyle's work involved not figuring out the entirety of Kosh's biology, but identifying the location and type of the poison. Once he had that, he neutralized it. There's little indication he could do a lot more, besides provide an optimal environment, and Kosh may have been able to recover at that point on his own.
Vorlon do have a physical aspect, although that doesn't necessarily mean things like walls pose much of a barrier. There are glimpses of Dr. Kyle's medical screens that mention things like crystalline structures, so he was clearly trying to learn and cram as much as he could.
Kosh still appears to have some form of consciousness even while affected. His suit opens after he senses Kyle's intents, and Lyta is sent directly to the information she is seeking as soon as she makes contact. Kosh could have been subtly helping him all along, nudging or highlighting ideas that would put him closer to helpful actions.