I'd just get something cheap.

That is a pretty large sprocket in the rear, and the 11T or 12T doesn't appear to have gotten much use, so you could go one size larger if you wish.

Just suggest to to your brother to replace the chain in a little less than 15 years next time.

I can't tell the wear on the chainring. Sometimes one has to replace the whole crankset. The worst wear will be perpendicular to the cranks (the part covered by the chain/chain guard in the photos.