Very loose serpentine belt
 

Hi guys. I let my sister borrow my car to drive to work and when I got it back she said that the battery light had come on. So I check it out and the belt has TONS of give and was flapping when I started it so I turned it off. Now I'm not very savvy when it comes to maintenance, but I would really like to save some cash and fix the problem myself. So I'm assuming I should replace the tensioner and the belt as well?

Sure sounds like the tensioner has let go, but I'd do a thorough inspection of the belt routing first. Maybe she ran through a puddle or off a ditch and knocked the belt loose. If the tensioner is shot, go ahead and replace the belt too. Check your battery voltage and make sure it's around 12 volts or the alternator won't pick it back up, you'll need to charge it.