Advice please,

I have an 87 240 wagon that ran great up until I took it into a mechanic yesterday. As part of preparing for a 300 mile trip to Miami I thought that it would be a good idea to have the belts changed. I had noticed that the ac belt was worn and as a precaution I would have ALL the belts changed. WELL, I received a call from the mechanic and he said that when he replaced the belts he noticed that they were spinning around the alternator and the alternator was locked up. It would not turn. He told me that an alternator replacement was necessary.I told him to replace it. Picked up the car. Readied for my journey, got onto the interstate. All of a sudden I had a stumbling motor, gauges not working, radio quit,etc,long list. I limped back home, turned off car,would not restart.Dead battery. Is it possible that a belt replacement could turn into a nitemare like this ??? What was suppossed to be a $100 repair cost me $600. Any Thoughts ???