The clutch itself can be replaced without losing the freon. Essentially you undo the center bolt and use a gear puller or pry the clutch off (after removing the belt). Mine, and maybe all, have a keyway, and the metal shims will come out with it.
Yes, you can replace just the clutch if you can find one, as Bob suggested above. You might want to consider though that if the compressor was old enough to have the clutch wear out that the rest of the compressor will not be far behind. A new clutch will run close to half the price of a new compressor and there is always the chance that the front seal in the compressor will not last all that much longer. Just a thought that you might want to just replace the whole compressor and get it over with.
I can sympathize, though, if he still has a good R12 charge, and doesn't have the technician's certificate to buy R12, or the money to invest in it. An R134a conversion can be done, but is undesirable if the R12 charge is still sufficient. I'd be tempted to change just the clutch in that case. If the charge is depleted enough, I'd convert.
A lot of the AC clutches go out because the front seal on the compressor leaks and causes oil to get on the plates. This leads to slipping, heat generation, destruction of the spring fingers or rivits due to chattering, and ultimately destruction of the hub bearing which will lock the hub up, even with the compressor off.
With that said, you can probably still find a complete clutch kit with the pulley (with bearing), hub plate, and coil. The use to sell for about $60 to $80. Just be forewarned that it won't last if the front seal is leaking.
For the clutch removal tool, you can make one from standard hardware parts. The instructions are on an Allpar page. AutoZone or Advance may have a loaner or one for sale fairly cheap.
Here is the Allpar link:
The best thing about Auto Zone is their loaner program. Advance doesn't have anywhere near the selection. I usually buy my parts at Advance and then pop over to AZ to get the tools.
Just because the clutch has failed doesn't necessarily mean that there's anything wrong with the compressor. If you're holding a charge, it's probably safe to say that you can just put a new clutch on and drive on. Any number of things could be wrong with the clutch, including coil and wiring failure. That being said, before I put the new clutch on, I would probe the connector for 12V with the engine running and the A/C on. If you have low/no charge, there will be 0V, as the pressure cutout switch will not allow the system to run with no refrigerant in it. If you have no refrigerant, it might be worth doing the R134a conversion since the system has already gone through the trouble of draining itself for you. That's a bit of a different animal. It can be DIY'd as long as you know what needs to be replaced.
The A/C has been converted to R134 and the clutch has not worked for 8 months but checked the compressor and it still has a full charge so I know the compressor is good. The biggest thing is replacing the clutch, I will check with autozone and see if they have the tools.
How did you determine that the clutch is bad? What electrical measurements did you get? Did you jump 12V directly from a battery to the clutch's terminals to see if it actuates? Otherwise, it could be a bad AC or fan relay, a low-pressure cutoff switch, a wiring problem, a climate control switch problem, etc.
Looks like Rockauto has the clutch available and has two part numbers that are on close out at a much lower than normal price. It might work out to be a good deal even with shipping costs. You will also need a shim kit and look at the shop manual for the shim and clutch gap set up. If the new clutch is not set up properly it will either not pull-in or will self destruct. Bob makes a good point about checking out the electrical issues first. You haven't mentioned why the clutch needs to be replaced is it a mechanical issue or is the clutch coil open? John also makes a good point about leaking front seals and burned clutches from oil getting on them. Sometimes taking what looks like the less expensive way out turns out to be the more expensive way- been there done that!