A friend just purchased a double at a gun show and there is a hole under the fore arm I am supposing where and when it was re-blued. Is this an issue that needs repairing?
Will it eventually be a problem for rust?

Many times a hole is drilled in the bottom rib to allow easy run out of the water during a re-rust blue. That water will usually get inbetwen the ribs even from tiny pin holes in the solder joints.
It has a real hard time getting back out.

Some bbls are drilled that way from the various factorys for the same reason.

What ever the source,,it doesn't cause any problems,,there is no real need to 'patch' it up unless the looks of it bothers you.
It won't cause a rust problem.

Most Win21's have 2 of these, one just forward of the forend hanger in the bottom rib. One in the short rib betw the hanger and the bbl flats.
Done at the factory. They are usually plugged afterwards by simply staking a lead BB into the hole with a punch.
It still shows of course, but it does plug the opening,, if that matters.

It’s also a great place to use a syringe to squirt a small amount of linseed oil into the interior to stop any rusting, just not too much.

You can push a piece of lead shot in the hole after oiling