"Can you do it?"
"Yes!"
Snape and Lupin had never exactly had a good relationship. They had even clashed many times back when they were students.
But now they were both mature adults, and both members of the Order of the Phoenix. If he could help Lupin, Snape would not deliberately make trouble over old conflicts. He would even, for Dumbledore's sake, do his best to ask Yinsen to lend a hand.
He had assumed Lupin's problem would be troublesome, perhaps requiring an "examination" to determine the details, followed by consideration of whether it could be treated. No matter what, he had never expected Yinsen to answer so readily and decisively.
"Um... shouldn't we examine Professor Lupin first?"
"An examination would be fine."
Yinsen did have many potions capable of changing Lupin's constitution, but most of the potions he possessed were meant to strengthen his own crew. A few had various effects such as detoxification and removing abnormal conditions.
Lupin's problem probably was not that serious. He only needed to remove the "toxin" that had invaded his body and altered his constitution.
The worst-case scenario was that his constitution had been completely transformed into that of a werewolf. Yet even that would not trouble Yinsen. He could simply reverse the condition—at his current level, he would not even need to prepare a potion. Magic alone could accomplish it.
Still somewhat uncertain, Snape went with Yinsen to find Lupin. Along the way, he also sent word to Dumbledore.
He was not worried that something would go wrong. Rather, he knew Lupin was still unaware of this matter. If Yinsen went to him directly, Lupin might suspect their true intentions. At a time like this, they needed someone respected and trustworthy like Dumbledore present.
"Not here."
They had only just met Lupin