Can't see any good reason to do this really, and that's coming from someone who's used electrical and gaffer tape (a.k.a. really good duct tape) on all sorts of modifications to gear for decades. I've even held a rusted-through car door together with gaffer tape.
As David said, it will leave an awful, sticky mess when it starts to come off, and I don't think even gaffer tape is going to stick very well to kydex in the first place. Gaffer/duct tape will eventually delaminate, even if it stays stuck. On my car, the tape stayed on for about two years but the top layer delaminated after a year to leave the underlying fibre layer in place.
How about an alternative if your friend really wants to cover the sheath... find some wide heat-shrink tubing, the type used for electrical insulation, and shrink that into place? Not sure you can get a Kermit the Frog version though, or Hello Kitty come to that... but who knows these days?