there are many shuttle bus from lhasa to rikazi, and it's only about 3-4 hours (the highway is best one in tibet). 12 hours, may be, if you via Lake YangZhuYong (jiangzi).
To Mt. Evenest, your should hire a car, and it's normally 4 days, and the fare is about 4000-5000RMB. It's impossible for only 15 hours if he spend 12 hour to Rikazi. It's a long way from Rikazi to Mt. Everest.
Sorry for my poor english, just because I can't use Chinese here.