Mercury - Sable

2000 Mercury Sable V-6 - car had been sitting idle for several months while
nephew was in the service. He changed the battery, but hooked up wires in
reverse. I changed back correctly and car will start but battery will
drain down and not charge. I think he may have fried the alternator, but
checking to see if it's possible a fuse or relay may be blown. Any ideas.

On this car, the alternator is on right side of engine,fairly low, so
doesn't look very easy to access. Any idea how much to change out if it's
bad?

Thanks for you