Your brother has damaged the AWD system on his Legacy by driving on mis-matched tires. Depending on the transmission type there are different fixes for this. Installing the correct size tires is not one of them, although he should do this anyway.
If it's a manual, tell him to drain and replace the transmission and differential lubricants. If he's lucky it might help.
If it's an automatic, he's probably going to need a new transfer valve and clutch pack. These components are located in the rear of the transmission. It cost me about $750 when I needed this for my '96 Legacy. The previous owner had been driving on mis-matched tires, and I didn't know it would damage the car.
I know it now, and I make sure all four tires are evenly matched and rotated on a regular basis.
If you want a cheap, temporary fix, insert a fuse in the AWD receptacle under the hood. It will disable the rear drive and the car won't be binding on turns. This only works on the automatic cars, and of course it will no longer have AWD, just FWD, but it will prevent internal transmission damage, which is what will happen if someone continues to drive the car the way it is.