Just one more thought:

I agree that the likelihood to any of the blocks (and thus transactions/coins) being valid is very low.

But the most likely ones to be valid are the earliest blocks mined. It is at least possible that your brother mined some valid blocks but then his network got isolated and the rest are orphans.

If the old client shows all transactions as "valid" and "mined" (like it seems to be the case according to the screenshots) then even if there are blocks which were "really" confirmed in the canonical Bitcoin blockchain, once there is one which was not and got "orphaned", it's (almost) impossible that a further block in the chain from this moment on was accepted, because all blocks contain a hash from the block before, so no block can be "exchanged".

So I would start the process of checking the blocks/transactions strictly chronologically, from first to last. Do it like Cricktor described here.