Alternator light came on last night. Took a chance and drove the car home with it on (for about 20 minutes). Found the serpentine belt shredded this morning. Got a mechanic to come change it and now it's squealing loudly. Squeal goes away with water for a few seconds. The mechanic says the alternator is getting really hot and that the alternator may need changing. Could it be that the alternator is overworking to recharge the battery? The alignment looks good and I hope the tension is good as well. The mechanic said the tension is good. Is it safe to drive the car with it squealing like that?

Sounds like it's locking up and that's what shredded your first belt. So, I agree with him. Replace the alternator.