The NXEA Trains Only routing is available on tickets from London Terminals but not from Boundary Zone 6.
looking at NFM09:
Boundary Zone 6 to Cambridge Any Permitted, 22.50, no restrictions (presumably limited by the Off Peak Travelcard though)
from LST to CBG
FDR - ANYTIME DAY 1R XEA
00020 - NXEA TRAINS ONLY
1 Adult @£ 17.10 = £ 17.10
__________
£ 17.10
Restriction : B6
OUTWARD TRAVEL
By any train except those timed to depart Mondays to Fridays before 0815
Am I reading this right? he can use the NXEA train from 0815, go first class (if there is first?) cheaper than the boundary extension?