It depends on what the current surface of the ball is and what kind of lane conditions you're bowling on.  What works for one bowler, ball, surface prep, and lane conditions may not work for another.

If the ball has a sanded finish OOB (anywhere from 320-grit to 4000-grit, which looks pretty shiny) then adding a grit-less polish will give a length and backend but possibly make it more susceptible to carrydown and spotty backends.  If the heads are drying out but there is still plenty of friction on the backend (I'm thinking wood lanes or Guardian) then adding polish to a sanded ball might be the right thing to do.

If the ball is fairly dull, increasing the grit of the surface without polish will also give length and backend but without some of the carrydown issues.  If the backends are getting spotty and the oil is moving around a lot, bringing up the surface to a higher grit can help get through now-spotty heads but not skate in the backend.  Also, the extra length and backend may allow you to move deeper to find more head oil and go around the carrydown in the back.

The reason the Fury Pearl has a 4000-grit finish rather than the similar Rough Buff finish (220-grit plus RB) is that while the compound RB finish gives length like the 4000-grit finish, in the end it's a polished finish that can be squirty on dirty backends (that sounds vaguely obscene).

But there's no one right solution.  As usual.

SH

It also depends on the polish you use.  Some polishes contain a slip agent that is designed to add length and reduce (or at least not increase) backend reaction.  Track's (Ebonite's?) Delayed Reaction and Legends Extended Length polish are like that.  You wouldn't normally use a polish like that unless you specifically wanted that type of reaction change, especially on top of an already-polished Twisted Fury (maybe I read it wrong and he had the regular Fury polished).

You could say that there's a bit of a gap between the Fury and the Twisted and that there's more than enough room to bring them closer together and still be useful or stuff another ball or two in between them.  If your friend doesn't want to buy more equipment, I'd take the Fury to something like 1500-grit or 2000-grit instead of the stock 800-grit.  That will keep it from hooking too early and make it more usable on more conditions.  He could hit it with a light coat of Brunswick's factory polish or give it an honest Rough Buff finish.  In both cases, though, I'd expect it to be pretty darn strong on the backend.

SH