It IS possible to encrypt a non system volume in place.

Windows? If not, it should be close to the same process, but you're on your own. I think it needs to be NTFS as well.

First, the VC container you now have on it will be double encrypted, the container file will remain there. So you may want to move your data out of the container first. Open it and copy the files out if you have the room. If not, you either have to leave it or get something else to copy the files to, then delete the container file.

In the main VC dialog, click TOOLS then VOLUME CONTAINER WIZARD then NEXT. Put the pip in the ENCRYPT A NON-SYSTEM PARTITION/DRIVE then click NEXT. STANDARD VERACRYPT VOLUME, then NEXT. Click SELECT DEVICE and choose your partition. Continue on through to do the encryption. I don't have one I can do now, so you will have to continue on from there, it should be logical. It will likely take a good long while, so plan accordingly.

As already mentioned by another poster, Windows is going to bitch about this partition and will assign it a drive letter. It will offer to format it. So you'd be best to help your bro out the first time and make sure you remove the drive letter from that partition using the Windows DISK MANAGEMENT tool on his computer, indeed, on ANY computer you use it on in fact. You can leave the drive letter, but any time you click on it, even accidently, Windows will offer to format it for you. ONE wrong click and it's kaput! So be careful, or like I said, remove the drive letter.