"I wouldn't be especially worried about availability on Beijing-Xi'an."
Problem is, if you're wrong -- and it does happen sometimes -- he's up the creek. Strongly suggest he put his mind at ease and get the tickets on ctrip beforehand.
When I know I have a trip coming up, I monitor availability on 12306 starting ~10 days out, and if things start to tighten up, I pull the trigger with ctrip. Most of my routes are not especially busy though.