Target disk Macbook Pro HD not showing on Host Macbook Pro
You mention "unlocking" the drive which would suggest your brother is using filevault (the drive is encrypted). If the drive/file system is indeed damaged, I find it unlikely you will be able to recover data. 

If your brother happens to be using an "old design" MacBook Pro with a non-ssd harddrive, you can still pull the drive out quite easily and connect it externally to your macbook to have a look without the motherboard being a factor. You will not loose warranty by doing this, but do consult  ifixit (the best source of Apple repair guides online) to do this properly.