simple stuff first, make sure you're turning it the right way (anticlockwise).
WD40 will do bugger all, plus-gas is a proper penetrating solution, try that or coke.
Get your mate to weld a spare BB tool to the end about 4 foot of (sturdy) steel rod, make sure the frame is secure and get bouncing on the end – lots of leverage is much more effective than percussive persuasion.