No, there isn't, the price is identical to the Gidea Park AAA Emerson Park ticket though (so there's no anomaly here; no revenue reason why the ticket should not be valid as a London - Emerson Park).
But plenty of revenue reasons why we need something more concrete (for your rationale) - as we do not want to OP/his friend to get a penalty fare between Upminster and Barking. I want you to be right (for the purposes of passenger rights) not me.
To illustrate my POV further:
The mapped routes for Gidea Park to London Terminals (in addition obviously to shortest route and direct trains) is map CO (which is great eastern main line, shown as just a single line Liverpool St to Shenfield).
Mapped routes for Emerson Park to London Terminals (in addition obviously to shortest route) are CO, or EA, or TS (for this route, EA is the same as CO, and TS is romford-emerson park-upminster-barking thence all permutations west of there that C2C serve from time to time (inc the little used gas factory spur).
Our ticket (gidea park to london terminals) is valid by map CO and is also available to Gidea Park. I do not believe we can infer map TS for a gidea park-London terminals.