Ditto what my buddy Wreck said and...

The metal rod would have to be very long. You would need to change the location of the servos so they are close to the fuselage. You would then need a bunch of supports for the wire all the way back to the control surface. The fuselage is sheeted and not all hardwood, so I have no idea where those supports could be mounted. Besides that, the pull pull tubes are already in the fuselage!

It's either going to flex or just bend if he uses a rod. It will be hard to get the throws needed for real 3D flying. There will also be more control slop with them rods, which is a big deal on these FunFly airplanes. It will also be heavier. The pull pull cables practically weigh nothing...A couple big metal rods really could add significant weight. The Top Cap ARF is not one of those "A little extra weight won't matter" type of airplanes...

So, tell your friend no. Tell him to get some help (from you!) and to get the pull-pull set up right. It's a little confusing the first time, but really easy once you get it figured out. Makes for a VERY secure linkage that just can't be beat...I don't think I'll ever go back to rods now.