Just purchased the $29 X3 upgrade. Now I want to purchase it for my brother, who is currently using 8.5. His PC isn't online. Is it easy to register offline? Do I have to download the whole thing again for him, or can it be installed from my download once the purchase is made?
 
Thanks.

There is no need to download the files again. Use the download files you already have. Just copy down the serial number and regcode for the new version from his store account and register with those credentials.

Technically you cannot share the software, You can install it on as many computers as you like but you can only be using one at a time, in other words it is your copy and it is illigal to give it away or sell it to anyone else. 
You don't need to ever download it again for your own use. You now have your serial and activation codes. Don't loose them. And don;t forget to download and install the X3e patch. 
 
You brother, if he is a registered owner of 8.5 can upgrade on his own, Go to the Cakewalk products link at the top of the page here and follow the links to upgrading. 
He will be given a new serial number and activation code, but he can use your version you downloaded to install. You don't need to be on line for Sonar. 

scook
There is no need to download the files again. Use the download files you already have. Just copy down the serial number and regcode for the new version from his store account and register with those credentials.

Excellent. Thanks Scook. Wasn't looking forward to another 3 hour downloading session.  Now to get my brother's account info so I can make the purchase for him.  (It'll be an early birthday present!)

If this $29 dollar deal is only plain jane X3 , not studio or Producer, and your brother is using 8.5 Producer, I don't think it's so much of and upgrade. He'll get to use his 8.5 vst's, but it's really a downgrade to go from producer to basic no matter which version is involved. I guess it still might be a cheaper upgrade path later on. 

I read that OP to infer that his brother was buying X3 but his computer wasn't on the 'net.  You CAN copy the install files to the new computer and just use your brother's newly purchased registration codes to register his copy of the software on his computer.