O2 sensor replacement HELP!!

I am working on a 2000 Ford Mustang V6 3.8 L for a friend. He came to me with the check engine light on. I scanned it and got a P0141, Oxygen sensor heater malifunction (bank 1 sensor 2). I am wondering if anyone can give me any info. on this so I know if I can do the job myself or not. Where is this sensor? I am guessing it is the one after the catalyst? Is it a pain to replace?
thanks!

Put the car on jackstands. Find a wrench to fit the O2 & dont round it off. It just unscrews from the exhaust pipe and should have a quick disconnect for the wiring. The reinstall of the new peice should be self explanatort. If not then do not attemp this simple task. Just FYI, O2 sensors are easy to round off if you are not careful.

Is it in a place where you can get enough leverage to break the O2 sensor off? I know sometimes they seemed to practically weld themselves on.

Well I'm not familiar with your exact application so all I know to do is to tell you to take a look for yourself.

Where do you live? Any car that resides anywhere there is salt used on the highway is prone to having the O2 sensor round off or beak before it comes off, unless heat is used. You may want to send that job to a mechanic. If he/she is nice they may let you provide your own sensor. Proof in point, I have a few 96 Crown Vic PI's . I recently started parting them out, the cars from around here (New York State) I either broke or rounded off so bad I couldn't remove the O2 sensors. I had 1 car from a sunny state, with just a little rust breaker, and minimul force I was able to remove all 4 for use in a project car. Oh a quick tip I found usefull. Unseat the pins from the plug, make sure you markthe colors on the plastic plug. Then I found you can use a box wrench with out having to buy one of those special sockets, that don't work anyway, they just bend..

I live around Chicago. So odds are then it will be rounded off. I think I will tell him to go to a mechanic. He doesn't seem to mind the check engine light being on. I told him it would just decrease his gas mileage and he said "That's why I've been filling up so much!"
Thanks for the help!