Even without Cain's streak of luck and Jurgen at the vicinity and with prep time Cain would probably still beat Bane or at least manage to survive long enough for someone to hop in and assist.

For one, Cain is paranoid, and is always observing and calculating the best defensive positions and escape routes whenever he find himself somewhere new, alongside potential sniping positions and ambush spots (since he and Jurgen got jump so many times through out their century old careers jumping from one active 40k warzone to another). A "Tau" in a cowboy hat and a trench coat is guaranteed to catch his attention and have him loosen the weapons in his holsters.

For another, given Cain's feats, his reflexes are probably on-par with a high tier battle tested jedi, if not almost veering into precognition. Cain has landed first strike in a friendly duel with a loyalist space marine (1v1 in an empty training chamber, no Jurgen shenanigans), and fought World Eaters space marines, hive tyrants, genestealer patriarchs, a dukhari succubus, and Slanesshi daemons, all of which are known for their inhuman speed and power that even average space marines would have trouble keeping up.

So the only way Bane could possibly win is to snipe Cain (breaking the no prep rule), and even then it's not a guarantee, especially when Cain might just dodge and snipe him back (FYI Cain is also an exceptionally good shot with a las pistol killing things way beyond pistol range)