It can be done in theory, as can most airplanes if you develop the attachment fittings and the airplane has a reasonably slow takeoff and landing speed. The Beech 18 has long been used on floats in the bush.
Notwithstanding the fact that it was done once upon a time on a Navion, in the absence of a pre-existing STC, and access to the particular floats covered by it, the costs would be staggering unless you put the airplane into the Experimental category and even there it would be only slightly less than staggering.
I don't see any signs of a float kit that was ever developed for the Navion for Edo or Wipline, and I assume it's a certfied category aircraft, so I'm going with starting from scratch, which means staggering, which means "fuggeddaboudit".
As an ex bush pilot, as far as doing well as a float plane, several comments:
As a thing, in general, low wing float planes suck. The wings get in the way of docks.
The takeoff and landing speed needs to be reasonably low, ideally below 50 kt. There is a bit of a hydrodynamic "wall" you hit around that speed while aquaplaning and if you can't lift off before that speed you zoom along seemingly forever building that last few knots, not to mention the concrete like character the water starts to take on.
To have good utility, unless you base it at a lake and only fly to other lakes, the floats should be amphibious, but amphibs on a 4 place airplane leave you with a 2 place airplane (even though you've removed the landing gear). Straight floats have only a minimal effect on useful load (because credit is taken for the lift generated by the floats plus the removal of the ldg gear), but amphibs are brutal because the weight of landing gear and pumps is ballast. In the bush you don't see that many working airplanes with amphibs for that reason unless they really need it.
Hull insurance on a straight floatplane is somewhat higher than wheels, but on an amphib, the premium will melt your eyeballs. Because, maybe sooner or maybe later, just about every single amphib out there will get landed wheels down on water, eventually.
The floats are destabilizing in yaw, and the Navion has a rather smallish rudder/fin, so it will almost certainly need a ventral fin. Expect to add it to the STC development work. Only a few sea planes have gotten away with not adding extra fin area.
So, bottom line is, unless you can find an installation STC, and can locate a set of floats covered by the STC, making it just bolt it all together, rig, make a log entry, and fly, it's a gargantuan task that will quickly bring your friend down to earth unless money is no object.