That's really something your friend needs to discuss with his/her physician. This shouldn't be a case where his/her physician (especially if a nephrologist) is completely unfamiliar with the amino acid.
Having said that, you're right citrulline is, in a way, a precursor to arginine in the urea cycle. I do know that arginine at least, has been looked at as a possible tool in certain forms of renal disease, at least in animal models, but officially, arginine is contraindicated in those with renal failure.
So, if it were me, I'd certainly avoid it, but again, this is something to be discussed with their physician.