I didn't realise that the whip could be used like that. Still, it feels wrong to me. That is not the same as it being overpowered or that anyone else ought to share those feelings.Rogues can already pick up a whip, tbf. I’m just proposing letting them have a 2hand option that is 1 die step higher.
And yeah, the polearm isn’t a big deal for Monks, one on one. It does let them have more targets with their many attacks, though, which is fun.
If the character seems like Indiana Jones I would even be fine with a rogue using the whip as a primary weapon.