If the doctors don't know what is causing kidney disease...?

Someone very close to me has a high creatitine reading of 4.0 which puts the kidney function at 17% and a biopsy was done and the rheumatologist was seen but lupus and vasculitis were ruled out and the nephrologist stated that this is a very unusual case and they cannot figure out what is causing this. I have the same blood type as this person but they won't do a transplant yet since they have to know the cause first or they said the new kidney can also fail if that risk is taken (not knowing what caused kidney disease). An MRI has been scheduled and they are going to try blood thinners in the event there is a possible clot that is causing the blood not to flow well to the kidneys. What I am concerned with is that if this is not the case and they cannot still find out what is causing this, does that mean this person won't survive despite dialysis until they let me donate my kidney?

I'm sorry your friend is dealing with this! However, even if the cause is not yet understood, there is no reason to suspect that the dialysis will not work. A healthy kidney is constantly cleaning/filtering the blood; dialysis is basically an artificial way of cleaning the blood when the kidney is not able to do it. The dialysis machine should be able to clean the blood whether or not the Dr's understand why the kidneys are not working.
Your friend is very lucky to have someone of the same blood type so willing to donate; however, please don't forget that blood type is not the only criteria. If your friend does need a transplant, the Dr's will need to do 'tissue typing' to make sure that you are a good and safe match.

sounds like you should be getting a second opinion ,instead of sitting around waiting for this Dr to decide what to do next=good luck