A/C Problems - '00 Sport

My brother has a '00 Dakota 4wd Sport and his A/C suddenly stopped working last Wednesday. He replaced a relay (got from the dealership), and it still doesn't work. I'm not sure if this is related in any way (doesn't hurt to give too much info I suppose), but his battery died on Tuesday. He replaced it with a new one (that day); the A/C worked that night, but the next day it didn't work at all.

I was wondering if anyone has had this problem and/or has a possible solution. Taking it to a dealership to get it fixed is a last resort (my friend will probably look at it first before he goes that route). Any help is greatly appreciated.

Just wanted to mention that I did search, but AF won't let me search for "A/C".