Generally, when you convert a system that was designed for R-12 to R-134a, the cooling capability is reduced. There are various ways to address this problem, the most common being to install a parallel-flow condenser (as opposed to the stock serpentine one), which aids in greater heat transfer. However, this requires custom lines, as PF condensers are generally universal, and no drop-in replacements likely exist for your vehicle.
With that said, this particular vehicle is about as good as it gets for a conversion. A relatively small cab to cool combined with a quite large surface area for the condenser makes the conversion pretty reasonable-performing without any of the above nonsense.
To perform the conversion, you're looking at disconnecting all the components, flushing all the lines, installing all new o-rings and adding back the appropriate amount of esther oil, adding r-134a adapter fittings to the service ports (required for proper conversion), and charging it with ~80% of the R-12 capacity (and adding a conversion label). On these vehicles, I frequently hear that going from the stock white orifice tube to a ford blue orifice tube also helps. Finally, you always replace the accumulator after having the system opened up.
Since it sounds like your compressor has seized, flushing everything is already a mandatory step before putting it back together. Pay close attention to the condenser, as you normally get about 75%+ of the debris in there after a compressor failure. On my '90 K2500, I replaced the condenser. I probably could've flushed it OK, but there were quite a few bent fins on it as well, which helped with the decision to replace. Nice thing about esther oil (or POE) is that it's compatible with both R-12 or R-134a, so you could go with either one (but don't mix them
).
Sounds like your buddy just had a straight up conversion. Having gotten in there and done this type of work myself, I'd say $500 is pretty reasonable. It's dirt cheap if that figures includes a replacement compressor; otherwise figure on another $200-300 for that.