It doesn't matter where the ESC is plugged into the receiver. All the + and - pins for the servos are wired together.

I must assume that he's got it plugged into the correct channel, or he wouldn't have throttle control.

What plane is it? That would help in determining the problem, as would more information on the battery. There is not much we can determine from the mAh rating. Is it NiCd? NiMH? What size cells? AA? AAA? 2/3A? It doesn't pay to be vague.