Just to clarify, you already have the nut off of it, right?....and it's the axle itself that is stuck inside the wheel?
Spray some penetrating lube in there and hit it with an impact wrench a few times at low pressure. A few sharp impulses will do better to break it free than trying to muscle it off with high force. Once you do, I'd suggest replacing it and anything else that was stuck to it. And when you reinstall make sure the axle is not dry, lube it lightly with something.
Its unlikely the axle isnt turning with the nut off because the only thing to hold it would be the brake carrier. Everyting else is bearings and with the nut relieved I doubt the spacers could grip it enough to cause the issue.
It happens but is rare and yes the penetrant impact way would be the way to go.
If he has the nut off then the issue is usually the axle wont slide out because of it being siezed/corroded to the spacers and/or bearings.
Many times when you try to hit the end of the axle, the outer arm thats recieving the impact (through the stuck axle/spacer assy) will act like a spring, absorb the impacts and bounce back.
Ive seen guys sledge the shiat out of axles and not get them out, because of that springing action. Sometime a strategically placed heavy piece of wood on the outer arm helps keep it from springing out and exerts more impact load in the axle. Just be careful to not ding up the arm.
I use a handheld airhammer and at its imact rate it hits while the arm is springing back. This has been the best way Ive found to unstick an axle.
Of course you never pound on the end of the axle and destroy the threads. Its best to use a sacrificial nut screwed onto the threads leaving it proud, or put the stock one on backwards at least.
If its not too stuck then a hammer and drift will work with some penetrant.
Either way let us know what it is/was, good luck and dont hurt her.