Originally Posted by
Stranger
Actually perhaps the most likely would be at at LHR the LH agent doesn't let him board to FRA. Then he would have to deal with the situation there.
You're just going to get a cursory examination at the gate in LHR by the LH agent there. That is probably the least likely location to get stopped.
The AC agent doing the doc check at point of departure (AC is responsible for you if you get denied entry) or the customs agent at point of entry will be the ones looking most closely at the passport.