My friend here at work has a 1999 Dodge Dakota 5.2L. He says every time he switches his A/C on, his fuse blows for the A/C system... Any ideas?

It could be a short in the compressor assembly, which it causing the fuse to blow every time it's switched on. Have a mechanic check the wiring and watch for a jump in voltage.

Where is the compressor assembly located?

Trucks and cars run on DC electricity (direct current) witch means that all electricity comes back to be grounded into the chassis of the truck, so you may be experiencing a short in your system like this8384 said, because that is the only thing I could think of that would make your fuse blow every time you turn it on... take it to a local shop you trust and have them look at the grounding wires from your air compressor, unless you can look yourself, hope this helps