Yes isn't totally unknown and in the deep vaults of BA FT there are a few previous cases like this. If HBO, mobile, on a quiet day and seemingly knows where to go, you can ask a supervisor to check with Control and the gate to see if they can still squeeze you on. In the past there are reports of people doing this (or not doing this) for Domestics in particular, when if someone is at North security, it's for example gate 6 departure, then clearly you don't need 35 minutes, or even 10 minutes to get to the gate. The system is not quite as ruthless as sometimes portrayed here. The underlying factor applies: they need people boarded in good time: having (e.g.) luggage on board and passengers not on board at gate C is woeful for all concerned.

The offloading is indeed automatic, but there is wriggle room for calls to be made. If your friend had no status, seemed unfamiliar with LHR, had luggage to check in and/or cost centres in tow, then I suspect a different answer would have applied. There is also a formalised and lower conformance at Flght Connections (down to 22 minutes at T5B to T5B). Best not to leave it too tight next time!

I think my narrowest conformance+security+gate is 6 minute to doors close (the CSD's hand was already on the handle as I approached). That was a bit of a rush.