If you can get the barrel off the best thing to do would be to go to the parts store and get a can of PB blaster. It is the best penatrating oil besides Kroil I've ever found.
If you can find it get the bulk can , not the spray can. Pour some in a bowl or heavy pastic cont. and let the barrel soak overnight. It should come right out.
You could also stick it in the freezer. The two metals shouldn't contract the same and sometimes that will free it up.
The most important thing to remember about this problem is prevention- buy a tube of anti-seize or some good quality Choke tube lube. Then you won't be faced with this issue.
Every time I clean my guns with screw in chokes, I remove the tubes, clean them well and coat the choke tube threads well before re-installation.
In the absense of something fancy like Kroll, use plain old kerosine. Pour enough kerosine into a small can to raise the level about 3 inches. Tie the shotgun (or just the barrel) so that the muzzle will hang down into the can of kerosine. Let it sit for several days and it should loosen it right up.
kroil is the stuff, pour some in a bowl and stand the barrel up and let soak overnight. Lessened learned...Don't let your friends borrow you guns. I have seen people actually weld a metal rod to the inside of the choke tube to actually be able to screw it out as an last resort, we lost the choke tube but at least we got it out.