Even without restoring to a wish, it seems that getting its hands on a magical weapon to sunder the +1 vorpal longsword would be pretty much automatic for a Pit Fiend.
Given the nature of devils to have long drawn out plans, I would suggest that the Pit Fiend is probably aware of a possible encounter with the Balor and would plan accordingly.
If I was the Pit Fiend I would offer advancement to the Balor's Marilith with some cunning deal (we are talking about devils after all) that would work to my advantage.
I could see this playing out in a campaign where it looks like the Balor has every advantage in the upcoming battle to be outdone, in the end, by the double-crossing devils.
I've always kind of had the idea that the reason the nine hells can battle the obviously much larger abyss and hold them to a stalemate is due to superior planning and tactics. I could even theorize that the strife between Graz'zt, Orcus, and Demogorgan are the result of hellish machinations (ala Asmodeus).