My friend sent his wife's passport to British embassy for stamping
transit visa. He has not yet received it. The flight is on Tuesday
Evening. He lives 8 hours away from NYC. Since the trip is urgent,
they don't want to cancel the flight. He is thinking about going
directly to embassy and enquiring about it. But he is worried that the
embassy may not give his wife's passport to him without her physical
presence. He would like to avoid taking his wife and child with him
and staying overnight and putting them to inconvenience.
Is there a gurantee that he will get the passport if he goes in person
to embassy without bringing his wife and child along with him?

Thanks in advance.

Phone the embassay first thing Monday morning and ask for advice.