The rifle needs attention and it needs it right now... If you can not get to the rifle like today, have your Grandfather take the barrel off and pour boiling hot soapy water through the barrel. This will at least flush a lot of the chemicals that are or might be working in the barrel right now. After that have him pour just boiling water through it to rinse the soap out of it. Then have him run a HEAVY SATURATED quality oiled patch through the barrel to stop any further action of the rust.

You still need to get to that rifle as soon as you can. The actions your Grandfather takes tonight will not save the barrel but will stop the damage from getting worse. When you get to it, do a entire water bath again just like your cleaning the gun after coming back from the range. Get a bore light and have a look down the barrel or if it is an in line look through the barrel. See what kind of damage if any there is...

If the barrel is pitted you have a couple choices. You can shoot it and see how bad it shoots. If it is acceptable, then learn to live with your mistake and do not make the same one again. If it is not the greatest, then you might try lapping the barrel. I am not saying it will save the rifle but at this stage of the game it can not hurt.