It can be noble or naive depending on how much consideration you has given to this. You should certainly consider all possible outcomes for yourself and weigh the risks vs benefits and then decide if you want to do it. Otherwise, just jumping into it because you want to help a friend would not be the best approach.
Anyway, no matter what, it's major surgery. That means being under general anesthesia, having open or laparoscopic surgery to remove the organ followed by recovery for probably at least a few days in the hospital, then likely weeks to months of limited activity recovery (ie, in bed, no heavy lifting, sports, work, etc). There will likely be pain during recovery which you'll take some meds for. Then also consider risks for complications of surgery (all the standard major risks for hemorrhage, clots, infections) that could cause problems requiring further medical treatment and/or potentially death.
There are also later risks associated with things like adhesions from surgery, the possibility of complications associated with a single kidney. Bladder infections, kidney infections and urinary tract infections in general have a higher likelihood of occuring just due to decreased urinary flow, but less likely in males than females as Dave rightly pointed out. Also, this will always be a consideration for medical treatment since the kidney is very important (you could say ultimately) in maintanence of fluids in the body, excretion of wastes and even medications. Nothing that one kidney can't handle, but should always be mentioned in future medical care. Also, in the case of any problems down the road, anything that could cause kidney failure would be more of a concern for you. If there is any history of diabetes in the family, you should certainly be sure to have regular medical care to screen for this as it can affect the kidney.
So, after all that, which I'm sure would be part of consent for the procedure, if you still feels you want to go through with it (and are least 18 years old), that is you decision and I commend you for it. But, there should be no shame in declining either. There is plenty that could go wrong both now and down the road with this and you should be prepared to deal with any consequences for yourself or the possibility that the transplant for your friend could ultimately fail, despite how well the surgeries go.
EDIT: I apologize, I misread this as your friend thinking about donating to another friend, when it was YOU considering donating to your friend. I've corrected what I said to make more sense. Don't get me wrong, I don't mean to discourage or scare you out of donating, but I feel you should understand what is involved with such a commitment. I think you are a great person for just considering it.
Here's websites which goes into more detail and have some good information:
Q & A for Kidney Donors
Living Donors Online! Living Kidney Donation
http://www.ucsfchildrenshospital.org/pdf/living_donor.pdf
