Thats a nice bad guy. The implications of such a cool dude are that PC's are going to want to learn how to do that trick. Saying its a "monster" power only is weak considering its being done by a low level human without magic.
That's cool. I would have no problem with the PCs trying to disarm every round. I even built this guy so that disarming him (or slashing his whip) would work. That's why he has the short sword.
As far as slashing his whip, this is how I would handle it.
1. Readied action: "I ready an action to slash his whip in two", which triggers when the slaver hits, I guess, so that it's taut and can be slashed.
2. If I describe the whip as being taut ("His whip wraps around your leg and slows you down"), then you can just cut it without needing the readied action.
3. Just a regular attack to slice it in two, though it's harder to do and I'd give a -2 to the attack roll. If the player describes something cool - "After he lashes out at me, I wrap the whip around my now-free hand and draw a dagger with the other, cutting the whip in two" - I'd erase the -2 penalty.
On any successful hit, the whip is cut and can't be used. You'd need a sharp weapon, of course, not just any weapon. A mace wouldn't do it.
That brings up another point about how I look at description and how it's important to me. If you describe "wrapping the whip around my PC's arm" I think I'd let you Pull him 1 square with a Str vs. Fort attack as just a move action. You can't normally pull a guy, especially one who's at range, but the fictional situation and how things are described makes this move make sense, makes it possible, so it should be allowed.
(I think a move action's okay because it's similar to shifting next to him, but slightly better so you have to make the attack. And on a failure he might pull
you or knock you prone.)