I like this idea. That probably would have been before the transformation. Russet's transformation is part of his character "secret".Thanks ! I was thinking that Russet and Jack be acquainted with each other originally from the feywild, and have become closer due to more recent events.
Jack loves playing cards and sometimes gets in over his head with betting. Due to his gambling debts, he has been cursed by a feywild kingpin to be forever chasing his losses. Fortunately, someone else has taken a liking to him and is working to bend or break the curse. But Jack is not aware of any of this — all he knows is he’s got a heap of gambling debts, and a lucky streak that will wipe them all out is ever just around the corner. He’s an eternal optimist and a degenerate gambler. Often his worst gambling losses involve him being framed for cheating, or at least that’s his story and he’s sticking to it.
How about if Jack had been captured by bounty hunters, and had attempted to make a run for it when his captors were taking him over a mountain pass? Jack and Russet met while both on the run, and realized that the very same bounty hunters Jack had just escaped from were the very same that were pursuing Russet. Together, they laid an ambush for the bounty hunters, planning to trap them in a treacherous part of the mountain pass where two glaciers fuse together. Hiding nearby, Jack used spells to cause a couple of the bounty hunters who'd been keeping watch to fall asleep, and then some sort of charm magic to instigate suspicion and bickering among the remaining bounty hunters. The resulting confusion allowed Russet an opportunity to prevail in a 4-on-1 battle against the ambushed vigilantes.
The ambush on the bounty hunters may have occurred either before or after Russet's transformation from brown-orange fur to white fur. If Jack and Russet teamed up before that, the transformative event might have involved both of them, and a similar transformation could somehow account for a heightening of Jack's magical abilities relating to water, ice, and snow.
If we want to combine the events, then we could make it as you said, but rather than prevailing against the remaining bounty hunters, Russet was forced to retreat into the mountains, luring the remaining bounty hunters with him. This allowed Jack to escape - he may have tried to help, but Russet was moving too fast, and a sudden blizzard made tracking him impossible. You returned back, and contacted his gang to inform them of his bravery and presumed death exposed in the blizzard.
Thornley their leader would have been distraught and initally highly suspicious of you. You'd win him over, of course, and he would insist you stay with them for a while to recover. The next morning, Russet returned, carrying his magical adamantine spear and his fur bright white.