JB bore paste is a good idea, but I found something that is not that abrasive. I use Blue Wonder. It comes in a tube and is very effective. I use it with 0000 steel wool. It can also be used on blued metal with not harm to the finish.

If your friend has really bad rust and even some nasty pitting, you may have to use the JB bore paste. If it is not that servere, then the BW will work fine.

Give it a try